Adding completion extension loading and execution logic (#833)

* Adding ICompletionExtension interface

* Adding extension loading and execution logic

* Fixing compilation error in VS 2017

* Using MEF for completion extension discovery

* using await on GetCompletionItems

* Adding an integration test for completion extension and update the completion extension interface

* Update the completion extension test

* Fix issues based on review comments

* Remove try/cache based on review comments, fix a integration test.

* More changes based on review comments

* Fixing SendResult logic for completion extension loading

* Only load completion extension from the assembly passed in, add more comments in the test

* Adding right assert messages in the test.

* More fixes based on review comments

* Dropping ICompletionExtensionProvider, load assembly only if it's loaded at the first time or updated since last load.

* Fix based on the latest review comments

* Adding missing TSQL functions in default completion list

* Update jsonrpc documentation for completion/extLoad

src/Microsoft.SqlTools.ServiceLayer/LanguageServices/Contracts/Completion.cs

@@ -5,6 +5,7 @@
 
 using System.Diagnostics;
 using Microsoft.SqlTools.Hosting.Protocol.Contracts;
+using Microsoft.SqlTools.ServiceLayer.LanguageServices.Completion.Extension;
 using Microsoft.SqlTools.ServiceLayer.Workspace.Contracts;
 
 namespace Microsoft.SqlTools.ServiceLayer.LanguageServices.Contracts
@@ -23,6 +24,13 @@ namespace Microsoft.SqlTools.ServiceLayer.LanguageServices.Contracts
             RequestType<CompletionItem, CompletionItem>.Create("completionItem/resolve");
     }
 
+    public class CompletionExtLoadRequest
+    {
+        public static readonly
+            RequestType<CompletionExtensionParams, bool> Type =
+            RequestType<CompletionExtensionParams, bool>.Create("completion/extLoad");
+    }
+
     public enum CompletionItemKind
     {
         	Text = 1,
@@ -45,6 +53,19 @@ namespace Microsoft.SqlTools.ServiceLayer.LanguageServices.Contracts
             Reference = 18
     }
 
+    public class Command
+    {
+        /// <summary>
+        /// The identifier of the actual command handler, like `vsintellicode.completionItemSelected`.
+        /// </summary>
+        public string CommandStr { get; set; }
+
+        /// <summary>
+        /// Arguments that the command handler should be invoked with.
+        /// </summary>
+        public object[] Arguments { get; set; }
+    }
+
     [DebuggerDisplay("Kind = {Kind.ToString()}, Label = {Label}, Detail = {Detail}")]
     public class CompletionItem
     {
@@ -74,5 +95,15 @@ namespace Microsoft.SqlTools.ServiceLayer.LanguageServices.Contracts
         /// resolve request.
         /// </summary>
         public object Data { get; set; }
+
+        /// <summary>
+        /// Exposing a command field for a completion item for passing telemetry
+        /// </summary>
+        public Command Command { get; set; }
+
+        /// <summary>
+        /// Whether this completion item is preselected or not
+        /// </summary>
+        public bool? Preselect { get; set; }
     }
 }