Support GCS files for extra_requirements argument in Python Beam portable runners

Currently Portable runners only support locally available files for adding dependencies on remote workers. This can be seen in https://github.com/apache/beam/blob/master/sdks/python/apache_beam/runners/portability/stager.py#L429 as it uses shutil.copyfile when it detects file is remote and it is not http.

An easy extension would be to extend _is_remote_path in Stager to detect if the path matches any filesystem and if it does the avoid downloading and let it be copied afterwards.

Acceptance criteria:

• `extra_package` can be a GCS path instead of requiring it to be local only.