move MonoPInvokeCallbackAttribute to a separate file

src/csharp/Grpc.Core/Internal/MonoPInvokeCallbackAttribute.cs

@@ -0,0 +1,40 @@
+#region Copyright notice and license
+
+// Copyright 2019 The gRPC Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#endregion
+
+using System;
+
+namespace Grpc.Core.Internal
+{
+    /// <summary>
+    /// Use this attribute to mark methods that will be called back from P/Invoke calls.
+    /// iOS (and probably other AOT platforms) needs to have delegates registered.
+    /// Instead of depending on Xamarin.iOS for this, we can just create our own,
+    /// the iOS runtime just checks for the type name.
+    /// See: https://docs.microsoft.com/en-gb/xamarin/ios/internals/limitations#reverse-callbacks
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Method)]
+    internal sealed class MonoPInvokeCallbackAttribute : Attribute
+    {
+        public MonoPInvokeCallbackAttribute(Type type)
+        {
+            Type = type;
+        }
+
+        public Type Type { get; private set; }
+    }
+}

src/csharp/Grpc.Core/Internal/NativeLogRedirector.cs

@@ -87,22 +87,4 @@ namespace Grpc.Core.Internal
             }
         }
     }
-
-    /// <summary>
-    /// Use this attribute to mark methods that will be called back from P/Invoke calls.
-    /// iOS (and probably other AOT platforms) needs to have delegates registered.
-    /// Instead of depending on Xamarin.iOS for this, we can just create our own,
-    /// the iOS runtime just checks for the type name.
-    /// See: https://docs.microsoft.com/en-gb/xamarin/ios/internals/limitations#reverse-callbacks
-    /// </summary>
-    [AttributeUsage(AttributeTargets.Method)]
-    internal sealed class MonoPInvokeCallbackAttribute : Attribute
-    {
-        public MonoPInvokeCallbackAttribute(Type type)
-        {
-            Type = type;
-        }
-
-        public Type Type { get; private set; }
-    }
 }