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.

Merge branch 'nicki/ci-shotgun-eval' into 'main'

See merge request isc-projects/bind9!10127