[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

net/FlatBuffers/FlatBufferConstants.cs

@@ -19,7 +19,7 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 
-namespace FlatBuffers
+namespace Google.FlatBuffers
 {
     public static class FlatBufferConstants
     {