Add support for ISO 8601 timestamps in syslogs

[JSCU-CNI]

This PR adds support for ISO 8601 timestamps in unix syslogs introduced in recent versions of Debian and Ubuntu distributions. The helper functions have been copied from #901. Once #860 is merged in main I propose to let the auth plugin import those functions from the helper file. Fixes #909.

[Poeloe]

Since the ISO timestamp format support has been merged to main for the AuthLogPlugin (#860), it's possible to add that support in this PR now.

@Schamper because I don't have a particular opinion about the placement of this helpers.py file, do you agree on its location?

[JSCU-CNI]

Thanks for your review @Poeloe. I have implemented your suggestions in 65548b9.

[Poeloe]

Could you also remove the RE_TS_ISO variable from the auth.py file and import the RE_LINE and RE_TS variable from the helpers.py file instead of defining it separately in the auth.py file?

[JSCU-CNI]

Implemented your suggestions in fc1471b.

[Poeloe]

Looks good!

[codecov]

Codecov Report

Attention: Patch coverage is 82.05128% with 7 lines in your changes missing coverage. Please review.

Project coverage is 77.89%. Comparing base (af1abe4) to head (5972340).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
dissect/target/plugins/os/unix/log/helpers.py	85.18%	4 Missing ⚠️
dissect/target/plugins/os/unix/log/messages.py	70.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #907      +/-   ##
==========================================
- Coverage   77.89%   77.89%   -0.01%     
==========================================
  Files         324      325       +1     
  Lines       27879    27888       +9     
==========================================
+ Hits        21717    21724       +7     
- Misses       6162     6164       +2     

Flag	Coverage Δ
unittests	77.89% <82.05%> (-0.01%)	⬇️

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