Tde migration integration (#1806)

Adding certificate migration for encrypted databases.

Uses notifications to send events back to the client every time each database certificate is migrated.

src/Microsoft.SqlTools.ServiceLayer/Migration/Models/StaticTokenCredential.cs

@@ -0,0 +1,35 @@
+//
+// 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.Threading;
+using System.Threading.Tasks;
+using Azure.Core;
+
+namespace Microsoft.SqlTools.ServiceLayer.Migration.Models
+{
+    /// <summary>
+    /// Temp token creadentials to interact with ArmClient class. 
+    /// The token passed to this class should be a newly request token, because this class doesn't renew the token.
+    /// Once MSAL is rolled out on ADS, we will implement a way to use the same ADS token cache configured by ADS. 
+    /// </summary>
+    internal class StaticTokenCredential : TokenCredential
+    {
+        private readonly AccessToken _token;
+
+        /// <summary>
+        /// Build credentials using a token that will not change.
+        /// </summary>
+        /// <param name="accessToken">Newly created token that should last for the duration of the whole operation.</param>
+        public StaticTokenCredential(string accessToken) => _token = new AccessToken(
+                   accessToken: accessToken,
+                   expiresOn: DateTimeOffset.Now.AddHours(1));  //Default to an hour, the current duration of a newly create token.
+
+        public override AccessToken GetToken(TokenRequestContext requestContext, CancellationToken cancellationToken)
+            => _token;
+
+        public override ValueTask<AccessToken> GetTokenAsync(TokenRequestContext requestContext, CancellationToken cancellationToken)
+            => new ValueTask<AccessToken>(_token);
+    }
+}