[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/Table.cs

@@ -18,7 +18,7 @@ using System;
 using System.Text;
 using System.Runtime.InteropServices;
 
-namespace FlatBuffers
+namespace Google.FlatBuffers
 {
     /// <summary>
     /// All tables in the generated code derive from this struct, and add their own accessors.