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/Completion/Extension/CompletionExtensionParams.cs

@@ -0,0 +1,32 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace Microsoft.SqlTools.ServiceLayer.LanguageServices.Completion.Extension
+{
+
+    [Serializable]
+    public class CompletionExtensionParams
+    {
+        /// <summary>
+        /// Absolute path for the assembly containing the completion extension
+        /// </summary>
+        public string AssemblyPath { get; set; }
+
+        /// <summary>
+        /// The type name for the completion extension
+        /// </summary>
+        public string TypeName { get; set; }
+
+        /// <summary>
+        /// Property bag for initializing the completion extension
+        /// </summary>
+        public Dictionary<string, object> Properties { get; set; }
+    }
+
+}

src/Microsoft.SqlTools.ServiceLayer/LanguageServices/Completion/Extension/ICompletionExtension.cs

@@ -0,0 +1,40 @@
+//
+// 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.ServiceLayer.Connection;
+using Microsoft.SqlTools.ServiceLayer.LanguageServices.Contracts;
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Microsoft.SqlTools.ServiceLayer.LanguageServices.Completion.Extension
+{
+    public interface ICompletionExtension : IDisposable
+    {
+        /// <summary>
+        /// Unique name for the extension
+        /// </summary>
+        string Name { get; }
+
+        /// <summary>
+        /// Method for initializing the extension, this is called once when the extension is loaded
+        /// </summary>
+        /// <param name="properties">Parameters needed by the extension</param>
+        /// <param name="cancelToken">Cancellation token used to indicate that the initialization should be cancelled</param>
+        /// <returns></returns>
+        Task Initialize(IReadOnlyDictionary<string, object> properties, CancellationToken token);
+
+        /// <summary>
+        /// Handles the completion request, returning the modified CompletionItemList if used
+        /// </summary>
+        /// <param name="connInfo">Connection information for the completion session</param>
+        /// <param name="scriptDocumentInfo">Script parsing information</param>
+        /// <param name="completions">Current completion list</param>
+        /// <param name="cancelToken">Token used to indicate that the completion request should be cancelled</param>
+        /// <returns></returns>
+        Task<CompletionItem[]> HandleCompletionAsync(ConnectionInfo connInfo, ScriptDocumentInfo scriptDocumentInfo, CompletionItem[] completions, CancellationToken cancelToken);
+    }
+}