feat(elasticsearch): L2 for ElasticsearchDomain (aws#8369)

I was using cdk to stand up an Elasticsearch domain and thought I might as well generalise the effort.

Can I please get some feedback on the current implementation and if it is something you are happy to accept?

This implementation is still missing tests, more documentation and features like importing the resource. I will add them once I know I'm on the right track.

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-elasticsearch/README.md

@@ -1,12 +1,163 @@
 ## Amazon Elasticsearch Service Construct Library
+
 <!--BEGIN STABILITY BANNER-->
 ---
 
-![cfn-resources: Stable](https://img.shields.io/badge/cfn--resources-stable-success.svg?style=for-the-badge)
+| Features | Stability |
+| --- | --- |
+| CFN Resources | ![Stable](https://img.shields.io/badge/stable-success.svg?style=for-the-badge) |
+| Higher level constructs for Domain | ![Experimental](https://img.shields.io/badge/experimental-important.svg?style=for-the-badge) |
+
+> **CFN Resources:** All classes with the `Cfn` prefix in this module ([CFN Resources](https://docs.aws.amazon.com/cdk/latest/guide/constructs.html#constructs_lib)) are always stable and safe to use.
 
-> All classes with the `Cfn` prefix in this module ([CFN Resources](https://docs.aws.amazon.com/cdk/latest/guide/constructs.html#constructs_lib)) are always stable and safe to use.
+> **Experimental:** Higher level constructs in this module that are marked as experimental are under active development. They are subject to non-backward compatible changes or removal in any future version. These are not subject to the [Semantic Versioning](https://semver.org/) model and breaking changes will be announced in the release notes. This means that while you may use them, you may need to update your source code when upgrading to a newer version of this package.
 
 ---
 <!--END STABILITY BANNER-->
 
+Create a development cluster by simply specifying the version:
+
+```ts
+import * as es from '@aws-cdk/aws-elasticsearch';
+
+const devDomain = new es.Domain(this, 'Domain', {
+    version: es.ElasticsearchVersion.V7_1,
+});
+```
+
+Create a production grade cluster by also specifying things like capacity and az distribution
+
+```ts
+const prodDomain = new es.Domain(this, 'Domain', {
+    version: es.ElasticsearchVersion.V7_1,
+    capacity: {
+        masterNodes: 5,
+        dataNodes: 20
+    },
+    ebs: {
+        volumeSize: 20
+    },
+    zoneAwareness: {
+        availabilityZoneCount: 3
+    },
+    logging: {
+        slowSearchLogEnabled: true,
+        appLogEnabled: true,
+        slowIndexLogEnabled: true,
+    },
+});
+```
+
+This creates an Elasticsearch cluster and automatically sets up log groups for
+logging the domain logs and slow search logs.
+
+### Importing existing domains
+
+To import an existing domain into your CDK application, use the `Domain.fromDomainEndpoint` factory method.
+This method accepts a domain endpoint of an already existing domain:
+
+```ts
+const domainEndpoint = 'https://my-domain-jcjotrt6f7otem4sqcwbch3c4u.us-east-1.es.amazonaws.com';
+const domain = Domain.fromDomainEndpoint(this, 'ImportedDomain', domainEndpoint);
+```
+
+### Permissions
+
+#### IAM
+
+Helper methods also exist for managing access to the domain.
+
+```ts
+const lambda = new lambda.Function(this, 'Lambda', { /* ... */ });
+
+// Grant write access to the app-search index
+domain.grantIndexWrite('app-search', lambda);
+
+// Grant read access to the 'app-search/_search' path
+domain.grantPathRead('app-search/_search', lambda);
+```
+
+### Encryption
+
+The domain can also be created with encryption enabled:
+
+```ts
+const domain = new es.Domain(this, 'Domain', {
+    version: es.ElasticsearchVersion.V7_4,
+    ebs: {
+        volumeSize: 100,
+        volumeType: EbsDeviceVolumeType.GENERAL_PURPOSE_SSD,
+    },
+    nodeToNodeEncryption: true,
+    encryptionAtRest: {
+        enabled: true,
+    },
+});
+```
+
+This sets up the domain with node to node encryption and encryption at
+rest. You can also choose to supply your own KMS key to use for encryption at
+rest.
+
+### Metrics
+
+Helper methods exist to access common domain metrics for example:
+
+```ts
+const freeStorageSpace = domain.metricFreeStorageSpace();
+const masterSysMemoryUtilization = domain.metric('MasterSysMemoryUtilization');
+```
+
 This module is part of the [AWS Cloud Development Kit](https://github.com/aws/aws-cdk) project.
+
+### Fine grained access control
+
+The domain can also be created with a master user configured. The password can
+be supplied or dynamically created if not supplied.
+
+```ts
+const domain = new es.Domain(this, 'Domain', {
+    version: es.ElasticsearchVersion.V7_1,
+    enforceHttps: true,
+    nodeToNodeEncryption: true,
+    encryptionAtRest: {
+        enabled: true,
+    },
+    fineGrainedAccessControl: {
+        masterUserName: 'master-user',
+    },
+});
+
+const masterUserPassword = domain.masterUserPassword;
+```
+
+### Using unsigned basic auth
+
+For convenience, the domain can be configured to allow unsigned HTTP requests
+that use basic auth. Unless the domain is configured to be part of a VPC this
+means anyone can access the domain using the configured master username and
+password.
+
+To enable unsigned basic auth access the domain is configured with an access
+policy that allows anyonmous requests, HTTPS required, node to node encryption,
+encryption at rest and fine grained access control.
+
+If the above settings are not set they will be configured as part of enabling
+unsigned basic auth. If they are set with conflicting values, an error will be
+thrown.
+
+If no master user is configured a default master user is created with the
+username `admin`.
+
+If no password is configured a default master user password is created and
+stored in the AWS Secrets Manager as secret. The secret has the prefix
+`<domain id>MasterUser`.
+
+```ts
+const domain = new es.Domain(this, 'Domain', {
+    version: es.ElasticsearchVersion.V7_1,
+    useUnsignedBasicAuth: true,
+});
+
+const masterUserPassword = domain.masterUserPassword;
+```