While not strictly required by the protocol,
it will make testing easier.

Fix and un-skip the test for _file_containing_extension
in python reflection service to refer to the correct proto
and expect the right object to be returned. Enable it too.

This will allow other transports with built-in security to use this
filter.

- this is both the point where we *know* we have a config (or not), and the
  last moment before wait_until_ready-ness is not an issue