Support for gpodder sync protocol

[axgfn]

gPodder is a popular desktop podcast app with its own sync protocol that has been implemented in multiple clients and servers. It would be nice if elfeed-protocol had support for it. In my case, I want to be able to sync elfeed with AntennaPod by way of the GPodder Sync Nextcloud app.

Thanks for all your hard work on elfeed-protocol, I've been using it daily for years now!