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/Management/Common/DbSize.cs

@@ -85,7 +85,7 @@ namespace Microsoft.SqlTools.ServiceLayer.Management
                     case SizeUnits.TB:
                         return sizeBase*1024.0*1024.0*1024.0*1024.0;
                 }
-                throw new InvalidOperationException("SR.UnknownSizeUnit(SizeUnit.ToString())");
+                throw new InvalidOperationException(SR.UnknownSizeUnit(SizeUnit.ToString()));
             }
         }
         #region Object Overrides