Add pull request configuration for buildkite pr bot (#1129)

Author: mrodm

.buildkite/pull-requests.json

@@ -0,0 +1,20 @@
+{
+   "jobs": [
+     {
+        "enabled": true,
+        "pipelineSlug": "elastic-package",
+        "allow_org_users": true,
+        "allowed_repo_permissions": ["admin", "write"],
+        "allowed_list": ["dependabot[bot]", "mergify[bot]"],
+        "set_commit_status": true,
+        "build_on_commit": true,
+        "build_on_comment": true,
+        "trigger_comment_regex": "^(?:(?:buildkite\\W+)?(?:build|test)\\W+(?:this|it))",
+        "always_trigger_comment_regex": "^(?:(?:buildkite\\W+)?(?:build|test)\\W+(?:this|it))",
+        "skip_ci_labels": [ ],
+        "skip_target_branches": [ ],
+        "skip_ci_on_only_changed": [ ],
+        "always_require_ci_on_changed": [ ]
+     }
+   ]
+}