Revert deprecation of Object::add_extension() usage

This reverts commit 14b997eea3c2a541fe842025b4f7ff76bd74fba9.
Its just not practical to use the Config API as it stands,
the add_extension() wrapper does more than just a Config->update().

Most use cases can be covered via YML, but any conditional
additions (e.g. in unit tests) can still benefit from the
add_extensions() shorthand.