Make it possible to access capabilities returned in response to the AUTHENTICATE command

This is a follow-up to https://github.com/chatmail/async-imap/issues/134 which is implemented in https://github.com/chatmail/async-imap/pull/135 for LOGIN command.

According to https://www.rfc-editor.org/info/rfc3501/#section-6.2.2 `AUTHENTICATE` command may return capabilities ("A server MAY include a CAPABILITY response code in the tagged OK response of a successful AUTHENTICATE command in order to send ..."), but there is no way to access them with async-imap.

We need an API similar to `login_with_capabilities()` added in https://github.com/chatmail/async-imap/pull/135, but for the `AUTHENTICATE` command.

Start by reading issue #134 and the implementation in pull request #135, especially the login_with_capabilities() API. Trace how AUTHENTICATE responses are handled, then add the analogous way to expose returned capabilities and verify that successful AUTHENTICATE calls provide them to callers.

Written by the indexing model from the issue text.