Add support for "custom remotes"

Add support for "custom remotes"

This will be helpful for the "ostree native container" work in
https://github.com/ostreedev/ostree-rs-ext/

Basically in order to reuse GPG/signapi verification, we need
to support adding a remote, even though it can't be used via
`ostree pull`.  (At least, not until we merge ostree-rs-ext into ostree, but
 even then I think the principle stands)