Support enum with fields in `Config::Input`

[ThierryBerger]

Fix https://github.com/gschup/ggrs/issues/40.

Follow up to https://github.com/gschup/ggrs/pull/41.

Currently just hacking stuff together, I dropped forbid_unsafe for my tests ; but ultimately the plan is to upstream that work to bytemuck.

Maybe https://jack.wrenn.fyi/blog/deflect/ contains helpful insights.

[ThierryBerger]

To properly test the enum handling, test_synchronize_p2p_sessions Should have its enum equivalent to call into poll_remote_clients which continues until it hits a CheckedBitPattern call.