keys: limit keytypes and validate uploads of the new keys API

split of from #8815 (closed)

• limit possible uploaded keytypes
• bring back key format validation

We used to validate the pgp key actually looked like an armored pgp block. We should bring this back and have a similar thing for the katzenpost keys.

The two things could probably be combined in a datastructure like this

{ keytype: ClassThatVerifiesIt, openpgp: PgpKey }

Not sure if we need to expose these as a config option. For now i would say no - we don't.