[9.18] new: ci: Add shotgun perf test of DoH GET to CI

Add performance tests of DoH using the GET protocol to nightly pipelines.

Backport of MR !9926

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

See merge request isc-projects/bind9!9940

.gitlab-ci.yml

@@ -1588,6 +1588,14 @@ shotgun:dot:
   when: delayed
   start_in: 5 minutes
 
+shotgun:doh-get:
+  <<: *shotgun_job
+  variables:
+    SHOTGUN_SCENARIO: doh-get
+    SHOTGUN_TRAFFIC_MULTIPLIER: 3
+  when: delayed
+  start_in: 5 minutes
+
 .stress-test: &stress_test
   stage: performance
   script: