Search SDK: Merging all v2.0-preview feature work to master branch (A…

…zure#469)

* Search SDK: Disabling synchronous methods for SearchIndexClient

Currently we are using the spec for SearchIndexClient to generate a proxy that
is then wrapped with hand-written code. The hand-written C# code always calls
async methods, so there is no need for the proxy to have synchronous methods.

* Search SDK: Modeling AnalyzerName and DataType as strong types

AnalyzerName and DataType are like enums, but they need to be open-ended, so
we have a custom type for them. It is better than a class of static string
properties since it facilitates type-safety and Intellisense (for .NET code).

* Search SDK: Adding new indexer features to 2.x-preview spec

* Search SDK: Turning off codegen for IndexingParameters

We need to mark a property of this class with the [Obsolete] attribute. We
need to do this by hand until this AutoRest issue is fixed:

Azure/autorest#1285

* Search SDK: Adding field mappings to API spec

* Search SDK: Modifying description of DataContainer.query

It now takes into account the existence of new datasource types.

* ETags for Azure Search Indexes, Indexers, and DataSources 🔎

* Search SDK: Adding support for custom analyzers

* Search SDK: Adding descriptions to some external types

search/2015-02-28-Preview/swagger/searchindex.json

@@ -5,7 +5,8 @@
     "description": "Client that can be used to query an Azure Search index and upload, merge, or delete documents.",
     "version": "2015-02-28-Preview",
     "x-ms-code-generation-settings": {  
-      "useDateTimeOffset": true  
+      "useDateTimeOffset": true,
+      "syncMethods": "None"  
     }  
   },
   "consumes": [