relation-list cannot list a remote app for a relation id

With the addition of <rname>-relation-created (https://github.com/juju/juju/pull/11341), it became apparent that there are cases where a unit is unable to list a remote app for a given relation id while handling an event without a relation context.

Example:

install
cluster-relation-created # JUJU_REMOTE_APP is set
ca-relation-created # JUJU_REMOTE_APP is set
leader-elected # <--- no relation context here but also no remote units observed
# ...

update-status # <--- no relation context here units may or may not be present

It is possible to retrieve a remote app for a peer relation without the relation context because it matches the app of a unit in question. The same is not the case for other relations as `relation-list -r <rid>` does not accept any parameters to list a remote application for a given relation id.

It would be good to have something like

relation-list -r <rid> --app

for cases like this.