add DefaultAuthority constant to channel options

src/csharp/Grpc.Core/ChannelOptions.cs

@@ -135,6 +135,9 @@ namespace Grpc.Core
         /// <summary>Initial sequence number for http2 transports</summary>
         public const string Http2InitialSequenceNumber = "grpc.http2.initial_sequence_number";
 
+        /// <summary>Default authority for calls.</summary>
+        public const string DefaultAuthority = "grpc.default_authority";
+
         /// <summary>Primary user agent: goes at the start of the user-agent metadata</summary>
         public const string PrimaryUserAgentString = "grpc.primary_user_agent";