Add support for pull_request_target webhook event

[gmlewis]

GitHub API documentation:
https://docs.github.com/en/actions/reference/events-that-trigger-workflows#pull_request_target

As mentioned in #1619, this webhook event needs to be supported.

This would be a great PR for any new contributor to this repo or a new Go developer.
All contributions are greatly appreciated!

Feel free to volunteer for any issue and the issue can be assigned to you so that others don't attempt to duplicate the work.

Please check out our CONTRIBUTING.md guide to get started.

Thank you!

[ganboonhong]

Hi @gmlewis, I would like to work on this. 😄
Could you please assign it to me? Thanks.

[gmlewis]

Thank you, @ganboonhong !
It is yours. 😁

[jhlegarreta]

@ganboonhong thanks for having addressed this 💯; @gmlewis thanks for the support 👍 !