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 48.42767% with 82 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.17%. Comparing base (c67c28b) to head (5fa173a).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/gardenlinux/features/cname.py	39.74%	47 Missing ⚠️
src/gardenlinux/features/metadata_main.py	0.00%	28 Missing ⚠️
src/gardenlinux/features/parser.py	81.25%	6 Missing ⚠️
src/gardenlinux/features/__main__.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #172      +/-   ##
==========================================
- Coverage   91.08%   86.17%   -4.92%     
==========================================
  Files          27       28       +1     
  Lines        1234     1345     +111     
==========================================
+ Hits         1124     1159      +35     
- Misses        110      186      +76     

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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.