Now that the authentication token is at least shared between the webapp, the `swh
web` cli and the deposit. It makes sense to move it in the authentication module.

It should be done as follows:

• Duplicate the code from swh.web.client to swh.auth and make it as an auth cli
• Deprecate the swh.web.client cli and/or make it depend on swh.auth [2]
• Update the documentation links with proper redirection to the newly documented cli

[1] Something like:

swh auth --client-id=swh-web

[2] swh.auth is rather lightweight so there is no issue making swh.auth a deps of swh.web.client