Update grpc to a universal binary (x86_64|arm64). Fix issues with hanging because we were not shutting down a channel.

com.unity.ml-agents/Plugins/ProtoBuffer/runtimes/osx/native/libgrpc_csharp_ext.bundle.meta

   validateReferences: 1
   platformData:
   - first:
-      '': Any
+      : Any
     second:
       enabled: 0
       settings:
         Exclude Win64: 1
         Exclude iOS: 1
   - first:
-      '': OSXIntel
+      : OSXIntel
-      '': OSXIntel64
+      : OSXIntel64
     second:
       enabled: 1
       settings: {}

com.unity.ml-agents/Runtime/Communicator/RpcCommunicator.cs

 
         /// The Unity to External client.
         UnityToExternalProto.UnityToExternalProtoClient m_Client;
+        Channel m_Channel;
 
         /// <summary>
         /// Initializes a new instance of the RPCCommunicator class.
                     Debug.Log($"Unexpected exception when trying to initialize communication: {ex}");
                 }
                 initParametersOut = new UnityRLInitParameters();
+                m_Channel.ShutdownAsync().Wait();
                 return false;
             }
 
         UnityInputProto Initialize(int port, UnityOutputProto unityOutput, out UnityInputProto unityInput)
         {
             m_IsOpen = true;
-            var channel = new Channel($"localhost:{port}", ChannelCredentials.Insecure);
+            m_Channel = new Channel($"localhost:{port}", ChannelCredentials.Insecure);
-            m_Client = new UnityToExternalProto.UnityToExternalProtoClient(channel);
+            m_Client = new UnityToExternalProto.UnityToExternalProtoClient(m_Channel);
             var result = m_Client.Exchange(WrapMessage(unityOutput, 200));
             var inputMessage = m_Client.Exchange(WrapMessage(null, 200));
             unityInput = inputMessage.UnityInput;
             try
             {
                 m_Client.Exchange(WrapMessage(null, 400));
+                m_Channel.ShutdownAsync().Wait();
                 m_IsOpen = false;
             }
             catch