Add support to read and write metadata for a GL canonical name

[NotTheEvilOne]

What this PR does / why we need it:
This PR adds support to read metadata, especially feature sets and platforms, without parsing data from the features directory.

Which issue(s) this PR fixes:
Fixes #154
Fixes gardenlinux/gardenlinux#3244
Related gardenlinux/gardenlinux#3243

Special notes for your reviewer:
This PR will be tagged 0.9.2. Please be aware that the metadata is subject to change.

I would like to suggest that @nkraetzschmar and I discuss the opportunity to extend and define a common "metadata" file to replace or extend files like *.release, *.requirements and the S3 metadata file.

[codecov]

Codecov Report

❌ Patch coverage is 67.81116% with 75 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.58%. Comparing base (8d6fb94) to head (447dacd).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
src/gardenlinux/features/__main__.py	40.29%	40 Missing ⚠️
src/gardenlinux/features/cname.py	70.52%	28 Missing ⚠️
src/gardenlinux/features/parser.py	81.25%	6 Missing ⚠️
src/gardenlinux/features/metadata_main.py	96.55%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #172      +/-   ##
==========================================
- Coverage   89.06%   86.58%   -2.48%     
==========================================
  Files          33       34       +1     
  Lines        1500     1648     +148     
==========================================
+ Hits         1336     1427      +91     
- Misses        164      221      +57     

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[NotTheEvilOne]

After discussion with @nkraetzschmar we decided to go with the os_release file and metadata for this feature as initial implementation. I'll adjust method names and functionality to reflect that.