RFC: feature/auth-source-pass branch integrating auth-source with the pass secret manager

[Ted Zlatanov]

Working with Damien Cassou and others, I've pushed a proposed series of
patches from https://github.com/DamienCassou/emacs/pull/1 to the branch
feature/auth-source-pass

>From the Emacs maintenance side, this code comes with tests. On the
auth-source library side, the code adds hooks to make future additions
to auth-source backends a bit easier.

The copyright assignments are in order so I believe this is OK to merge.
Please comment on the code, if you have any questions or concerns.

Thanks!
Ted