accounts: add auth attrset in order to add more auth methods in the future

[blaggacao]

Description

This PR should lay the foundation for adding new auth mechanisms (such as Gmail OAuth) in the module's current shapeliness (i.e. without adding arbitrary auth-related attributes to the top-level namespace).

This shapeliness will make it obvious and (once the necessary migration of current modules is accomplished) cheaper to implement new auth mechanisms on the generic accounts interface.

It appears that previous efforts were frustrated by overwhelming complexity. This PR tries to split up the problem into smaller chunks, and specifically ensure migration of existing modules is completed by the time it can be undrafted.

Ref: #4219 & #4988

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all
  or nix build --reference-lock-file flake.lock ./tests#test-all using Flakes.

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Maintainer CC

[blaggacao]

@rycee I already got some of the necessary refactorings done locally. But before I charge on and involve the maintainers of aerc, neomutt and the likes to help review them: do you agree with this approach in principle?