[9.18] new: ci: Run shotgun tests on MRs

Execute DNS Shotgun performance tests on the regular MRs and compare the changes they introduce against the MR diff base. The results are evaluated automatically - the shotgun jobs will fail if thresholds for CPU/memory/latency difference is exceeded.

Backport of MR !10127

Merge branch 'backport-nicki/ci-shotgun-eval-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10184