[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.Benchmarks/FlatBufferBuilderBenchmark.cs

@@ -17,7 +17,7 @@
 using BenchmarkDotNet.Attributes;
 using MyGame.Example;
 
-namespace FlatBuffers.Benchmarks
+namespace Google.FlatBuffers.Benchmarks
 {
     //[EtwProfiler] - needs elevated privileges
     [MemoryDiagnoser]