Boostrap swh-auth module for authentication

We started implementing keycloak authentication in the deposit [1]

While the initial attempt is working, it's not enough. To iterate over this work, that will need some more code which is actually already present in swh-web.

So we need to bootstrap a new module swh-auth which moves that common code there.

We (@anlambert and i) see 2 modules for it:

• swh.auth which generically wraps the keycloak dependency collaborator classes/functions
• swh.auth[django] Optional dependency on django

Bonus point for moving the current swh-web-client cli which generates token in that new module ( but that's not an immediate concern).

Deliverable:

• 1 new module (with shared code and pytest fixture(s)): swh.auth, swh.auth[django])

• jenkins builds (tests, pypi, debian)

• 2 debian packages (python3-swh.auth and python3-swh.auth.django)

• [1] swh/devel/swh-deposit!399 (closed)

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Migrated from T3079 (view on Phabricator)