PyPI: API skeleton and authentication

Proposal

Implement the basic API skeleton for PyPI with authentication.

End points

# upload endpoint
POST gitlab.com/api/v4/projects/:id/packages/pypi 

# download endpoints
GET gitlab.com/api/v4/projects/:id/packages/pypi/simple/<package-name>
GET gitlab.com/api/v4/projects/:id/packages/pypi/<file-identifier>

Authentication

Authentication uses basic HTTP auth so it should be straightforward to support CI tokens or personal tokens.

See #34161 (closed) and epic &2554 (closed) for details