[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/Program.cs

@@ -19,7 +19,7 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Reflection;
 
-namespace FlatBuffers.Test
+namespace Google.FlatBuffers.Test
 {
     static class Program
     {