[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

tests/FlatBuffers.Test/FlatBuffersExampleTests.cs

@@ -21,7 +21,7 @@ using MyGame.Example;
 using optional_scalars;
 using KeywordTest;
 
-namespace FlatBuffers.Test
+namespace Google.FlatBuffers.Test
 {
     [FlatBuffersTestClass]
     public class FlatBuffersExampleTests