[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/ByteBuffer.cs

@@ -51,7 +51,7 @@ using System.Buffers.Binary;
 #warning ENABLE_SPAN_T requires UNSAFE_BYTEBUFFER to also be defined
 #endif
 
-namespace FlatBuffers
+namespace Google.FlatBuffers
 {
     public abstract class ByteBufferAllocator
     {