Add support for action `enable auto merge`

[Bluebugs]

Is your feature request related to a problem? Please describe.
When having rulesets defined for a branch a PR will wait for that PR to pass all the rules before it can get merged. There would not be any reason to run gitstream much more once the PR is approved if it was able to enable auto merge instead of waiting to do it manually once the ruleset resolve. It can also happen that a ruleset changing state and getting resolved does not trigger an event that can be caught by gitstream having enable auto merge would also solve that.

Describe the solution you'd like
Add a new core action: auto-merge@v1

Describe alternatives you've considered
I can add a github action that enable auto merge automatically, but that defeat the purpose of gistream. I can also add a new workflow that get triggered by gitstream. There is tons of way to do it, but none are really clean as it would with a new core action.

[vim-zz]

Hey, thank you for the suggestion, did you try using the merge action?

[Bluebugs]

Yes and in some case, it won't pick up the latest change to the check (I believe because they are happening outside of action and there would be no github event associated that would trigger a refresh of gitstream workflow). This seems to be mostly an issue when using ruleset. I have not really digged in which ruleset is causing that issue.

[vim-zz]

there is a known limitation with supporting rulesets which can explain what you see

[Bluebugs]

For reference an example of the problem with just standard action: mountain-reverie/playwright-ci-go#31 . As you can see even it was approved, but it didn't get merged as at the time the CodeQL check hadn't been pushed and when it eventually was, it didn't trigger any event ( https://github.com/mountain-reverie/playwright-ci-go/pull/31/checks?check_run_id=42416561914 ).

Having just enable auto merge as an action would just solve this issue. And there is other use case, as auto merge is something devs have to click and some time it could just be automated following certain rules.