Add SYNC_ASSETS_HOOK support

[marmarek]

This is useful when having site-local cache shared among several worker
hosts. See added documentation for scenario details.

Question: Should this setting be also exclusive to workers.ini, similar to GENERAL_HW_CMD_DIR and few others?

[github-actions]

Great PR! Please pay attention to the following items before merging:

Files matching docs/*.asciidoc:

• Consider generating documentation locally to verify it is rendered correctly using tools/generate-docs

This is an automatically generated QA checklist based on modified files.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.99%. Comparing base (5f42d4f) to head (6960512).
Report is 57 commits behind head on master.

Additional details and impacted files

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  Coverage   98.99%   98.99%           
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  Files         396      396           
  Lines       39611    39618    +7     
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+ Hits        39211    39218    +7     
  Misses        400      400           

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[marmarek]

ping?

[perlpunk]

I would suggest to add that new variable here: https://github.com/os-autoinst/openQA/blob/master/lib/OpenQA/Worker/Job.pm#L240
We don't want people to add that setting to a job; it's supposed to be only set by the workers.ini, right?

[marmarek]

Sure I can add it there. I included question about it in the PR description:

Question: Should this setting be also exclusive to workers.ini, similar to GENERAL_HW_CMD_DIR and few others?

TBH, I'm not sure what is the rule what settings are allowed only in workers.ini and what can be set via jobs. There are many variables that can take paths that may or may not be specific to individual worker filesystem layout (like several of GENERAL_HW_*_CMD, UEFI_PFLASH_*, EXTERNAL_VIDEO_ENCODER_CMD, just to name a few), yet only some of them are limited to workers.ini.

[perlpunk]

It's possible that there are more variables that should be only allowed via workers.ini.
I already talked about that in the past, and I think it's a bit unfortunate that all are ending up in the same variable.
Anything that can mess with the host system running qemu should probably forbidden.
Scheduling jobs cannot be done by any random person, so there is some protection, but another layer of protection can't hurt, especially since we can schedule jobs via PR comments now.

[perlpunk]

I created #6160