Resolve "Automatically add a milestone for MR"

What does this MR do and why?

This MR adds a new function to Triage-ops. This new function is automatically adding milestone for MR which is merged and does not has a milestone. It will select a milestone by merge commit sha and ref branches, adding this milestone to MR.

Screenshots or screen recordings

How to set up and validate locally

You can use bundle exec rspec to validate locally.
You can also use "gitlab-triage" and GDK to validate this new function.

MR acceptance checklist

• Add jihu_milestone_helper with relative codes to Triage.
• Add test case for new function.
• Add configuration file to make sure schedule mode can work.
• Validate locally.

Closes #22 (closed)
related issue : #22 (closed)