[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/ByteBufferUtil.cs

@@ -16,7 +16,7 @@
 
 using System;
 
-namespace FlatBuffers
+namespace Google.FlatBuffers
 {
 	/// <summary>
 	/// Class that collects utility functions around `ByteBuffer`.