Add AQL Assessment service (#946)

[SQL Assessment API](https://docs.microsoft.com/en-us/sql/sql-assessment-api/sql-assessment-api-overview) provides a mechanism to evaluate the configuration
of SQL Server for best practices. SQL Assessment API gives a list
of recommended actions to improve SQL Server performance or security.

The SQL Assessment service is used by the expected SQL Assessment
feature of Azure Data Studio. 

SqlAssessmentService forwards JSONRPC calls to SQL Assessment engine
and wraps results as a response.

`assessment/getAssessmentItems` returns a set of checks 
applicable to a given target.

`assessment/invoke` returns a set of recommendations
for improving SQL Server instance or database configurations. 

`assessment/generateScript` returns a T-SQL script for storing
an assessment result set to a SQL data table.

src/Microsoft.SqlTools.ServiceLayer/SqlAssessment/Contracts/GetAssessmentItemsRequest.cs

@@ -0,0 +1,33 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using Microsoft.SqlTools.Hosting.Protocol.Contracts;
+
+namespace Microsoft.SqlTools.ServiceLayer.SqlAssessment.Contracts
+{
+    /// <summary>
+    /// Parameters for executing a query from a provided string
+    /// </summary>
+    public class GetAssessmentItemsParams : AssessmentParams
+    {
+        // a placeholder for future specialization
+    }
+
+    /// <summary>
+    /// Describes a check used to assess SQL Server objects.
+    /// </summary>
+    public class CheckInfo : AssessmentItemInfo
+    {
+        // a placeholder for future specialization
+    }
+
+
+    public class GetAssessmentItemsRequest
+    {
+        public static readonly RequestType<GetAssessmentItemsParams, AssessmentResult<CheckInfo>> Type =
+            RequestType<GetAssessmentItemsParams, AssessmentResult<CheckInfo>>.Create(
+                "assessment/getAssessmentItems");
+    }
+}