[9.18] chg: ci: Only run relevant CI jobs based on the changes

Trigger selected CI jobs on MR automatically only if there are related
code changes. Otherwise, offer an option to run the jobs manually in
MRs. For other sources, like schedules, tags etc., execute the jobs as
usual.

Backport of MR !10987

Merge branch 'backport-nicki/ci-restrict-rules-changes-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10993