Rename C# files to indicate generated code (#10801)

* Change C# generation script to use .pb.cs extension

* Rename generated C# files

This was performed by running and then removing the old files.

- This does not change the conformance test generated C# code
- This does not change the compatibility C# code
- There's currently no clean-up operation in to remove old .pb.cs files (which would now be feasible)
- The changes to TestMessagesProto2.pb.cs are just due to formatting and a new Objective-C option

* Fix well-known type source files for C# Bazel build
diff --git a/csharp/.gitignore b/csharp/.gitignore
index 62c9638..56ac149 100644
--- a/csharp/.gitignore
+++ b/csharp/.gitignore
@@ -33,3 +33,8 @@
 # Benchmark output
+# Reinstate generated code for test files