Extend github-export-checks to include merge rule from ExecuTorch (#2000)

Summary:
Pull Request resolved: #2000

This adds a `merge_rules.yaml` file for ExecuTorch and extend github-export-checks to block diff when it fails

Reviewed By: bigfootjon

Differential Revision: D53839467

fbshipit-source-id: 638265b8eaf13d6e71f325419113c637c1ff204e

Author: huydhn

.github/merge_rules.yaml

@@ -0,0 +1,9 @@
+- name: superuser
+  patterns:
+  - '*'
+  approved_by:
+  - pytorch/metamates
+  mandatory_checks_name:
+  - Facebook CLA Check
+  - Lint
+  - pull