new: ci: Run an additional respdiff job for merge requests and schedules

On MRs it uses the merge target as the reference.
In schedules it uses the latest released version for this branch as the reference.

This MR lays the ground work for using respdiff on non-standard configurations (like ECS) in the public repo, see https://gitlab.isc.org/isc-private/bind9/-/merge_requests/807#note_573140.

To reduce the future hassle when maintaining the -S version, most of the work (including an added job, so we know that it actually works) is done here.

Merge branch 'stepan/respdiff-against-merge-target-or-last-release' into 'main'

See merge request isc-projects/bind9!10664