Add a generic Django REST Framework authentication backend enabling to
authenticate a user using Keycloak and OpenID Connect bearer tokens.

The backend can be easily plugged into a DRF application by:

  * adding "swh.auth.django.backends.OIDCBearerTokenAuthentication"
    to the REST_FRAMEWORK["DEFAULT_AUTHENTICATION_CLASSES"]
    django setting.

  * configuring Keycloak URL, realm and client by adding
    SWH_AUTH_SERVER_URL, SWH_AUTH_REALM_NAME and SWH_AUTH_CLIENT_ID
    in django settings

Users will then be able to perform authenticated Web API calls by
sending their refresh token in HTTP Authorization headers.

Related to T3150