[Migrate actions] Create merge branch action

[mikealfare]

Description

As a maintainer, I use a temporary branch when building releases, doing version bumps, building changelogs, etc. I need to merge that branch when I'm done.

Acceptance criteria

Beta Give feedback

1. I can call this action to merge a branch into a target branch in GitHub (not local)
2. I need the resulting SHA associated with the merge

Impact to other teams

This impacts Adapters and Core since they are the primary consumers of these actions.

Proposals

• named github/branch/merge
• inputs source-branch,target-branch,message
• outputs sha

[mikealfare]

We have a PR with these contents already.