[C#] Prepares for official Nuget release (#7496)

* Define nuget package

* C# Switch to Google.FlatBuffers namespace

* Add Source Link for nuget package

* Add Strong Name signing of Google.FlatBuffers

tests/FlatBuffers.Test/FlatBuffersTestMethodAttribute.cs

@@ -16,7 +16,7 @@
 
  using System;
 
-namespace FlatBuffers.Test
+namespace Google.FlatBuffers.Test
 {
     [AttributeUsage(AttributeTargets.Method)]
     public class FlatBuffersTestMethodAttribute : Attribute