bug#64202: [PATCH] Gnus: Add back end for Atom feeds (nnatom)

[Daniel Semyonov via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Attached is an updated version of the patch.

Changes:
- Add support for multiple authors/contributors.
- Change how links are parsed to avoid looping over all items in an
  article for both links and authors.
- Optimize the creation of a list of entries from a feed.

I also forgot to mention I expanded the info node in the last update.

At this point, other than optimizations and bug fixes, the backend is
basically complete; it has very good support for the Atom Syndication
Format, and (I think) it should be possible to implement any Gnus
backend as an inheriting backend of it, without changing any backend
interface functions other than possibly 'nnatom-request-type' and
'nnatom-request-rename-group' (a backend like nnimap will need to add
some backend functions though).

The nnyt backend I posted in my last message still works with this
version (although it does some unnecessary work now).

Apologies if you're not interested in this and you're CC'd; I just
copied the CC header from Eli's last message here since I don't really
know who works on Gnus.

Regards,
Daniel