Agent Proxy and Credential request handlers (#637)

* Agent Proxy account WIP

* Fixup Credential create\update\delete handlers

* Use current user for test credential

* Cleanup and delete code

* Convert tabs to spaces

src/Microsoft.SqlTools.ServiceLayer/Security/Contracts/CredentialInfo.cs

@@ -14,8 +14,9 @@ namespace Microsoft.SqlTools.ServiceLayer.Security.Contracts
     /// </summary>
     public class CredentialInfo
     {
-        public string Identity { get; set; }
         public int Id { get; }
+        public string Identity { get; set; }
+        public string Name { get; set; }
         public DateTime DateLastModified { get; }
         public DateTime CreateDate { get; }
        public string ProviderName { get; set; }