Add automerge pipeline

[ifosch]

Implementing pull request workflow using GitHub's auto-merge feature. The problem with this GH feature is that it has to be activated manually for every PR. So this action will automatically enable auto merge for every PR when:

• it is opened
• it is re-opened
• it is changed from "draft" to "ready for review"

If it's a draft PR or auto-merge is already enabled, this action won't run.

After that, the branch protection rules will ensure that:

• the PR has at least 2 approvals
• there are no requested changes
• the branch is mergeable

References:

• Blog post suggesting this action step
• GH's auto-merge feature
• GH's branch protection rules
• GH's docs on the pull request object