Add manifest units to expand output (New)

[Hook25]

Description

We need a way to programmatically fetch the list of manifest entries needed to run a test plan. This makes it easier to automatically verify manifests in CI, track changes to testplans manifest entries and more. This adds manifest units to the expand command filtering out those that aren't used in the testplan.

Resolved issues

Fixes: https://warthogs.atlassian.net/browse/CHECKBOX-1555

Documentation

This adds plenty comments to the expander.

Tests

WIP

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 2 lines in your changes missing coverage. Please review.

Project coverage is 45.88%. Comparing base (0881101) to head (570c8c6).
Report is 35 commits behind head on main.

Files with missing lines	Patch %	Lines
checkbox-ng/checkbox_ng/launcher/subcommands.py	88.88%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1489      +/-   ##
==========================================
+ Coverage   45.87%   45.88%   +0.01%     
==========================================
  Files         367      367              
  Lines       39156    39167      +11     
  Branches     6622     6628       +6     
==========================================
+ Hits        17961    17970       +9     
- Misses      20504    20505       +1     
- Partials      691      692       +1     

Flag	Coverage Δ
checkbox-ng	68.14% <88.88%> (+0.01%)	⬆️

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[pieqq]

Thanks for the comments in _get_relevant_manifest_units, otherwise that would be pretty complicated to parse!

I've played with it a little bit and it seems to do what it's advertised for.

I only have one inline comment about the fact that now every job has a template_id attribute, even if it's null.

[pieqq]

Thanks!