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 generate_protos.sh and then removing the old files.

Notes:
- 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 generate_protos.sh 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/generate_protos.sh b/csharp/generate_protos.sh
index cfd6082..8be4428 100755
--- a/csharp/generate_protos.sh
+++ b/csharp/generate_protos.sh
@@ -32,6 +32,7 @@
 # descriptor.proto and well-known types
 $PROTOC -Isrc --csharp_out=csharp/src/Google.Protobuf \
     --csharp_opt=base_namespace=Google.Protobuf \
+    --csharp_opt=file_extension=.pb.cs \
     src/google/protobuf/descriptor.proto \
     src/google/protobuf/any.proto \
     src/google/protobuf/api.proto \
@@ -52,6 +53,7 @@
 $PROTOC -Isrc -Icsharp/protos \
     --experimental_allow_proto3_optional \
     --csharp_out=csharp/src/Google.Protobuf.Test.TestProtos \
+    --csharp_opt=file_extension=.pb.cs \
     --descriptor_set_out=csharp/src/Google.Protobuf.Test/testprotos.pb \
     --include_source_info \
     --include_imports \
@@ -75,4 +77,5 @@
 
 # AddressBook sample protos
 $PROTOC -Iexamples -Isrc --csharp_out=csharp/src/AddressBook \
+    --csharp_opt=file_extension=.pb.cs \
     examples/addressbook.proto