[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/Offset.cs

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-namespace FlatBuffers
+namespace Google.FlatBuffers
 {
     /// <summary>
     /// Offset class for typesafe assignments.