Add non-UI config option to allow attended minor core updates

Problem/Motivation

For Core MVP we had initially planned to only support patch level updates.

I chatted with @xjm recently who suggested we could have a non-UI config setting to allow minor updates.

I think this sounds great.

Currently we disallow updates that have database updates required during cron. We should not allow minor updates via cron. They will likely need database update anyways.

Proposed resolution

1. add allow_core_minor_updates config switch. This switch is core specific to allow core have a different switch once we support config updates in core
2. Add warning at the start of minor update with reasons why the user should be more careful. Is there an existing documentation page for this?
3. Change \Drupal\automatic_updates\Validator\UpdateVersionValidator to allow minor updates if config is set and not in cron
4. Add warning in update apply form if an update DB update is detected.
5. re-route the user to update.php after the update if there are DB updates.

4 and 5 are existing problems because only stop updates with DB updates in cron so we should solve those first.