Cosmos DB - Add Support for AAD authentication

[ealsur]

This PR expands the support for authentication to include AAD.

Users can define on the Cosmos DB Trigger or binding a Connection value that points to a configuration that either:

• Is a string and represents a valid Cosmos DB Connection String

{
        "MyConnection": "<cosmos db connection string>"
}

• Is a JSON node with valid AAD information:

{
	"MyConnection" : {
             "accountEndpoint": "https://myCosmosDBaccount.documents.azure.com:443/",
	     "clientId": "AAD client Id",
	     "tenantId": "AAD Tenant Id",
	     "clientSecret": "AAD secret"
       }
}

[brettsam]

A couple of minor comments....

[pakrym]

LGTM