pipeline: outputs: elasticsearch: add api key as valid auth type

Adds documentation for the HTTP API Key authentication type to the
elasticsearch output plugin

Signed-off-by: Todd Kennedy <95242064+tkennedy1-godaddy@users.noreply.github.com>

Author: tkennedy1-godaddy

pipeline/outputs/elasticsearch.md

@@ -29,6 +29,7 @@ operational Elasticsearch service running in your environment.
 | `Cloud_Auth` | Specify the credentials to use to connect to Elastic's Elasticsearch Service running on Elastic Cloud | _none_ |
 | `HTTP_User` | Optional username credential for Elastic X-Pack access | _none_ |
 | `HTTP_Passwd` | Password for user defined in `HTTP_User` | _none_ |
+| `HTTP_API_Key` | API key for Elastic. If `HTTP_User` or `Cloud_Auth` are defined, this will be ignored  | _none_ |
 | `Index` | Index name | `fluent-bit` |
 | `Type` | Type name | `_doc` |
 | `Logstash_Format` | Enable Logstash format compatibility. This option takes a Boolean value: `True/False`, `On/Off` | `Off` |
@@ -56,7 +57,7 @@ be compared to the `database` and `table` concepts.
 
 ### TLS / SSL
 
-The Elasticsearch output plugin supports TLS/SSL. 
+The Elasticsearch output plugin supports TLS/SSL.
 For more details about the properties available and general configuration, see [TLS/SSL](../../administration/transport-security.md).
 
 ### `write_operation`