Add CI pipeline for testing Elasticsearch changes in serverless

.buildkite/check-es-serverless.yml

@@ -0,0 +1,9 @@
+steps:
+  - trigger: elasticsearch-serverless-update-submodule
+    label: ":elasticsearch: Check elasticsearch changes against serverless"
+    build:
+      message: "Validate latest elasticsearch changes"
+      env:
+        ELASTICSEARCH_SUBMODULE_COMMIT: "${BUILDKITE_COMMIT}"
+        UPDATE_SUBMODULE: "false"
+

catalog-info.yaml

@@ -31,3 +31,34 @@ spec:
         daily promotion:
           branch: main
           cronline: '@daily'
+---
+# yaml-language-server: $schema=https://gist.githubusercontent.com/elasticmachine/988b80dae436cafea07d9a4a460a011d/raw/e57ee3bed7a6f73077a3f55a38e76e40ec87a7cf/rre.schema.json
+apiVersion: backstage.io/v1alpha1
+kind: Resource
+metadata:
+  name: buildkite-pipeline-elasticsearch-check-serverless-submodule
+  description: Validate elasticsearch changes against serverless
+  links:
+    - title: Pipeline
+      url: https://buildkite.com/elastic/elasticsearch-check-serverless-submodule
+spec:
+  type: buildkite-pipeline
+  system: buildkite
+  owner: group:elasticsearch-team
+  implementation:
+    apiVersion: buildkite.elastic.dev/v1
+    kind: Pipeline
+    metadata:
+      description: ':elasticsearch: Validate elasticsearch changes against serverless'
+      name: elasticsearch / check serverless submodule
+    spec:
+      repository: elastic/elasticsearch
+      pipeline_file: .buildkite/check-es-serverless.yml
+      branch_configuration: main
+      teams:
+        elasticsearch-team: {}
+        everyone:
+          access_level: READ_ONLY
+      provider_settings:
+        build_branches: false
+        build_pull_requests: false