Add `mwoauth` crate
This adds helpers to implement the MediaWiki OAuth2 authentication flow.
It aims to be simpler than generic crates like oauth2
and agnostic to
your webserver (e.g. not Rocket-specific like rocket_oauth2
).
At some point this should be integrated into mwapi
or possibly mwbot
to automatically refresh the access_token when the API let's us know it
has expired.
Bug: T350997