Allow skipping the who-resolved info in slack posts

[tyrken]

What this PR does

All resolved alerts currently get an extra line of to say what resolved the alert (for us usually the alert source rather than a person), which I think not normally needed, and anyway repeated in the Plugin UI or the message log inside the Slack thread. So to save scroll-space on the Slack UI I'm proposing to remove the it. Implemented as a FEATURE_SKIP_SLACK_RESOLVED_BY for now, but TBH I'd prefer it to default True as I can't see it's that useful.

There is one problem though - when combined with #4206 this means resolved alerts have no buttons or other Slack Attachments, so there is no green coloured sidebar to highlight this is a resolved alert group. Only the lack of buttons & a red sidebar, which might be enough but not perfect. In our home-grown equivalent to OnCall we post slack messages as all Attachments and no Blocks to solve this, as AFAIK Slack do not allow adding a coloured sidebar to Blocks.

I'd obviously be happy to switch the existing Slack title/message Blocks over to Attachments in this or another PR.

Which issue(s) this PR closes

Further work towards #4165

Checklist

I've not added any extra tests/docs yet, awaiting feedback on whether this feature is acceptable & in what form you'd like extra work before doing so.

• Unit, integration, and e2e (if applicable) tests updated
• Documentation added (or pr:no public docs PR label added if not required)
• Added the relevant release notes label (see labels prefixed w/ release:). These labels dictate how your PR will
  show up in the autogenerated release notes.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[github-actions]

This pull request has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in 30 days if no further activity occurs. Please feel free to give a status update now, ping for review, or re-open when it's ready. Thank you for your contributions!