`forge pr list --head owner:branch` does not apply `--head` with gitea/forgejo

[untitaker]

As discovered in #109, --head is not actually implemented for gitea. So this is silently incorrect. I think we can improve upon the status quo:

• Attempt to upstream APIs https://codeberg.org/forgejo/forgejo/issues/12837
• Print warning about ignored flags
• Filter clientside

The same thing with --base afaict (though I haven't tested). There actually is a way to get specific PR from forgejo when both base and head are known, so that part could be implemented properly.

[untitaker]

The same issue is with the --author filter. It seems that forgejo v16 (unreleased) implements all of the relevant filters: https://v16.next.forgejo.org/api/swagger#/repository/repoListPullRequests

but this doesn't exist on codeberg.org and is not implemented in the gitea SDK. I'd have to use the raw HTTP client.

Thoughts?