[Ops] Create emergency serverless release testing pipeline (#170863)

## Summary
This pipeline job runs sanity checks for changes on a one-off branch
(such as a serverless emergency-release) and creates serverless docker
images required for deployment.

Closes: https://github.com/elastic/kibana/issues/170828

Pipeline created by:
https://github.com/elastic/kibana-buildkite/pull/130

.buildkite/pipelines/es_serverless/emergency_relelease_branch_testing.yml

@@ -0,0 +1,17 @@
+# https://buildkite.com/elastic/kibana-serverless-emergency-release-branch-testing
+
+## Triggers the artifacts container image build for emergency releases
+agents:
+  queue: kibana-default
+
+notify:
+  - slack: "#kibana-mission-control"
+    if: "build.state == 'passed' || build.state == 'failed' || build.state == 'scheduled'"
+
+steps:
+  - trigger: "kibana-artifacts-container-image"
+    label: ":docker: Build Kibana Artifacts Container Image"
+    build:
+      branch: $BUILDKITE_BRANCH
+      commit: $BUILDKITE_COMMIT
+      message: Running PR build for $BUILDKITE_BRANCH