feat(cron): Emit event before/after background job execution

[ChristophWurst]

• Required for feat(transaction): Track background jobs ChristophWurst/nextcloud_sentry#707

Summary

Emit events before and after running background jobs.

TODO

• Implement
• Test

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[juliusknorr]

Tested and works for me as well 👍

Anything missing still before moving out of draft state @ChristophWurst ?

[ChristophWurst]

I wasn't sure if including job arguments is needed or not, but that can be added later too

[ArtificialOwl]

Tested and works for me as well 👍

Anything missing still before moving out of draft state @ChristophWurst ?

haven't tested, code looks good.

In case of doubt, we provided the NCU\ namespace for new public interface:
https://docs.nextcloud.com/server/latest/developer_manual/digging_deeper/api.html#php-unstable-api

[ChristophWurst]

The API is forward-compatible for extension, we don't need the experimental namespace here