platform automerge only when Renovate checks passing

[rarkins]

Describe the proposed change(s).

Today, platformAutomerge=true means Renovate enables platform automerge at PR creation time. This is meant to mean "automerge this once required checks pass". However some projects have insufficient checks and the platform can automerge even if Renovate has posted failing checks such as renovate/artifacts.

It is proposed to modify platform automerge behavior so that:

• It's only enabled at PR creation time if all renovate checks pass
• If it was enabled at PR creation time, and at a later time then Renovate checks are not passing, then platform automerge should be disabled
• If platform automerge for a PR is disabled (either due to non-passing checks previously, or was not configured at the time) and then later it's enabled in config and with passing checks, then Renovate should enable it

Pseudo logic:

1. Suppress enabling platform automerge at PR creation time if any Renovate checks are non-passing
2. Record in PR body debug data whether we enabled or not platform Automerge (if platformAutomerge=true). No additional data if platformAutomerge=false
3. If in future platformAutomerge=true, checks are passing, and the PR debug data is missing platformAutomerge settings or has its state to false, then set platform automerge and update PR body debug data to state true
4. If in future platformAutomerge != true, or checks are not passing, and PR debug data indicates state is true, then we should disable PR platform automerge

We want to prevent the scenario where we enable platform automerge, then a user disables it, and then Renovate keeps re-enabling it. This scenario should be prevented by point 3 above, because in such a scenario the existing PR debug data for platform automerge will remain as state=true to remind us that we enabled it previously