Remove/deprecate use of HsClientIntroAuthKey from most/all contexts

This key type is not used in the protocol as implemented today. If we were to implement it, we'd probably want to reconsider the protocol described in rend-spec, and maybe even pick a different key type. (I would probably advocate for a dual-use curve25519 key, if we can be confident that it's safe.)

As such, it doesn't make a lot of sense to have HsClientIntroAuthKey in tor-hsclient.

cc @gabi-250 for thoughts