chore: add issue comment action

[zirain]

see: https://github.com/jpmcb/prow-github-actions

enable /assign /unassign milestone slash commands

[github-actions]

🚀 Thank you for contributing to the Envoy Gateway project! 🚀

Before merging, please ensure to follow the process below:

1. Requesting Reviews:

• cc @envoyproxy/gateway-reviewers team for an initial review.
• After the initial review, reviewers should request the @envoyproxy/gateway-maintainers team for further review.

2. Review Approval:

• Your PR needs to receive at least two approvals.
• At least one approval must come from a member of the gateway-maintainers team.

NOTE: Once your PR is under review, please do not rebase and force push it. Otherwise, it will force your reviewers to review the PR from scratch rather than simply look at your latest changes.

What's more, you can help expedite the processing of your PR by

• Ensuring you have self-reviewed your work according to the project's Contribution Guidelines.
• If your PR addresses a specific issue, make sure to mention it in the PR description.
• Respond promptly if there are any test failures or suggestions for improvements that we comment on.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (64d7152) 64.54% compared to head (675bfda) 64.54%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2285      +/-   ##
==========================================
- Coverage   64.54%   64.54%   -0.01%     
==========================================
  Files         112      112              
  Lines       15949    15949              
==========================================
- Hits        10295    10294       -1     
- Misses       5007     5008       +1     
  Partials      647      647              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[Xunzhuo]

Does this have access control ? Like who can use /milestone ?

Would like to enable:

            /assign 
            /unassign 
            /area 
            /kind 
            /priority 
            /remove 
            /close 
            /reopen 
            /lock 
            /milestone 
            /hold 
            /cc 
            /uncc

[Xunzhuo]

@zirain Have you tested it in your fork ? I worried about if it will work