[Backport 8.10] Run Buildkite CI on pull requests

Author: pquentin

.buildkite/pull-requests.json

@@ -0,0 +1,9 @@
+{
+  "jobs": [
+    {
+      "enabled": true,
+      "pipeline_slug": "elasticsearch-py-integration-tests",
+      "allow_org_users": true
+    }
+  ]
+}