Abolish `HsSvcKeyRole`

The following discussion from !1677 (merged) should be addressed:

• @Diziet started a discussion: (+3 comments)

  This is quite the generics, isn't it. Rhetorical question: why is it necessary? I think the answer is so that something can be generic over different key roles. (The alternative being to have a separate struct for each role, and never to reify the role other than as in an ArtiPath.) But I think I may be missing what...

  Does this need to be in the public API of tor-hsservice? I think not. Nothing other than tor-hsservice needs to handle HsSvcKeySpecifier? So can we make it crate-local ?

  Suggested disposition: for now, make this pub(crate). File a ticket and assign it to me, asking me to try out / demonstrate what the alternative looks like.