Enable ssh key registration via id token claim

[DrDaveD]

There are very likely going to be cases where ssh public keys should be supplied through the VO instead of self-registration. I believe that could be done through id token claim mapped to metadata that htgettoken sees. It may be sufficient if htgettoken then disallows the --registerssh option if that metadata is seen, while htvault-config continues to allow self-registration which htgettoken just does in a different way (that is, by passing in the public key metadata from the id token).

[DrDaveD]

Another option that was suggested was to update the vault ssh authentication plugin to be able to directly read ssh keys from LDAP.