Generate the well-known types in C#

This involves:
- Specifying a namespace in each proto (including ones we'd previously missed)
- Updating the generation script
- Changing codegen to implement IReflectedMessage.Fields explicitly (a good thing anyway)
- Changing reflection tests to take account of the explicit interface implementation

Non-generated code in this commit; generated code to follow

1 files changed, 1 insertions, 1 deletions

diff --git a/src/google/protobuf/field_mask.proto b/src/google/protobuf/field_mask.proto
index ad7a1dfa..c19f4410 100644
--- a/src/google/protobuf/field_mask.proto
+++ b/src/google/protobuf/field_mask.proto
@@ -34,7 +34,7 @@ package google.protobuf;
 option java_multiple_files = true;
 option java_outer_classname = "FieldMaskProto";
 option java_package = "com.google.protobuf";
-option csharp_namespace = "Google.Protobuf";
+option csharp_namespace = "Google.Protobuf.WellKnownTypes";
 option objc_class_prefix = "GPB";