a missing features for GLRD and gardenlinux's tests

[yeoldegrove]

What this PR does / why we need it:

In the latest refactoring a few features got lost that GLRD and gardenlinux's tests are currently depending on.

This PR adds those features back:

• optionally fetch flavors.yaml from GitHub
• build cached S3 index of objects add S3ObjectIndex Class
• parse gardenlinux Versions - TODO

[NotTheEvilOne]

Thanks for the changes @yeoldegrove. Do you mind splitting this PR for:

• "GitHub" functionality
• S3 extension

[codecov]

Codecov Report

❌ Patch coverage is 95.45455% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.32%. Comparing base (7933a0f) to head (01fda54).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
src/gardenlinux/flavors/__main__.py	95.23%	1 Missing ⚠️
src/gardenlinux/s3/bucket.py	93.75%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #132      +/-   ##
==========================================
+ Coverage   72.73%   73.32%   +0.59%     
==========================================
  Files          27       27              
  Lines        1192     1226      +34     
==========================================
+ Hits          867      899      +32     
- Misses        325      327       +2     

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[ByteOtter]

lgtm so far, most gripes I have are nitpicks about code style that we can address later with CI.
Only thing I would like to have changed is the name of the test function in test_bucket as its currently not being executed as far as I see because pytest can't detect it without the test_ prefix.
Fell free to treat all other comments as optional.

[ByteOtter]

When it comes to other test coverage, I can bring that back up later :)

Removing review as during absence I took over @yeoldegrove work and it looks like I can't remove the "request changes" status of the first review otherwise.

[ByteOtter]

While the handling of file operations causes me a slight tummy ache. I trust your judgement and approve this.
Maybe we can revisit this later.