Add GitHub Actions reusable workflow for @ghstack merge

[seemethere]

Adds a reusable workflow that can be called from other repositories
to automate ghstack landing via PR comments.

Files:

• .github/workflows/ghstack-merge.yml: Reusable workflow with inputs:

  • pull_request_number (required)
  • python_version, ghstack_version, runs_on (optional)
  • validate_rules, dry_run, comment_on_failure (optional booleans)

• .github/workflows/ghstack-merge-on-comment.yml: Example caller
  workflow that triggers on "@ghstack merge" PR comments

• .github/merge_rules.yaml: Example merge rules configuration

Usage in other repos:
uses: ezyang/ghstack/.github/workflows/ghstack-merge.yml@master
with:
pull_request_number: ${{ github.event.issue.number }}
secrets:
github_token: ${{ secrets.GITHUB_TOKEN }}

[seemethere]

Stack from ghstack (oldest at bottom):

• Add tests for merge rules validation engine #321
• Add example workflows and documentation for ghstack merge #320
• -> Add GitHub Actions reusable workflow for @ghstack merge #319
• Add --validate-rules, --dry-run, --comment-on-failure to land command #318
• Add merge rules engine for validating PRs before landing #317