[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/FlatBufferBuilder.cs

@@ -23,7 +23,7 @@ using System.Text;
 /// @addtogroup flatbuffers_csharp_api
 /// @{
 
-namespace FlatBuffers
+namespace Google.FlatBuffers
 {
     /// <summary>
     /// Responsible for building up and accessing a FlatBuffer formatted byte