Always Encrypted Azure Key Vault Support (#934)

Add support for running queries that require a decryption key from Azure Key Vault when using Always Encrypted.

src/Microsoft.SqlTools.ServiceLayer/Connection/Contracts/SecurityTokenRequest.cs

@@ -0,0 +1,50 @@
+using Microsoft.SqlTools.Hosting.Protocol.Contracts;
+
+namespace Microsoft.SqlTools.ServiceLayer.Connection.Contracts
+{
+    class RequestSecurityTokenParams 
+    {
+        /// <summary>
+        /// Gets or sets the address of the authority to issue token.
+        /// </summary>
+        public string Authority { get; set; }
+
+        /// <summary>
+        /// Gets or sets the provider that indicates the type of linked account to query.
+        /// </summary>
+        public string Provider { get; set; }
+
+        /// <summary>
+        /// Gets or sets the identifier of the target resource that is the recipient of the requested token.
+        /// </summary>
+        public string Resource { get; set; }
+
+        /// <summary>
+        /// Gets or sets the scope of the authentication request.
+        /// </summary>
+        public string Scope { get; set; }
+    }
+
+    class RequestSecurityTokenResponse
+    {
+        /// <summary>
+        /// Gets or sets the key that uniquely identifies a particular linked account.
+        /// </summary>
+        public string AccountKey { get; set; }
+
+        /// <summary>
+        /// Gets or sets the access token.
+        /// </summary>
+        public string Token { get; set; }
+    }
+
+    /// <summary>
+    /// SecurityToken Request mapping entry 
+    /// </summary>
+    class SecurityTokenRequest
+    {
+        public static readonly
+            RequestType<RequestSecurityTokenParams, RequestSecurityTokenResponse> Type =
+            RequestType<RequestSecurityTokenParams, RequestSecurityTokenResponse>.Create("account/securityTokenRequest");
+    }
+}