remote-add: Default to explicit sign-verify backends

remote-add: Default to explicit sign-verify backends

In https://github.com/ostreedev/ostree/pull/2092/commits/588f42e8c64183dfa1fbaa08cc92c46b691b23c4
we added a way to add keys for sign types when doing
a `remote add`, and in https://github.com/ostreedev/ostree/pull/2105
we extended `sign-verify` to support *limiting* to an explicit
set.

This PR changes the *default* for `remote add` to combine
the two - when providing an explicit `--sign-verify=type`,
we now limit the accepted types to only those.