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/HostLoader.cs

@@ -26,6 +26,7 @@ using Microsoft.SqlTools.ServiceLayer.QueryExecution;
 using Microsoft.SqlTools.ServiceLayer.SchemaCompare;
 using Microsoft.SqlTools.ServiceLayer.Scripting;
 using Microsoft.SqlTools.ServiceLayer.Security;
+using Microsoft.SqlTools.ServiceLayer.SqlAssessment;
 using Microsoft.SqlTools.ServiceLayer.SqlContext;
 using Microsoft.SqlTools.ServiceLayer.Workspace;
 
@@ -129,6 +130,9 @@ namespace Microsoft.SqlTools.ServiceLayer
 
             ExternalLanguageService.Instance.InitializeService(serviceHost);
             serviceProvider.RegisterSingleService(ExternalLanguageService.Instance);
+            
+            SqlAssessmentService.Instance.InitializeService(serviceHost);
+            serviceProvider.RegisterSingleService(SqlAssessmentService.Instance);
 
             InitializeHostedServices(serviceProvider, serviceHost);
             serviceHost.ServiceProvider = serviceProvider;