Add function to git checkout a certain commit and parsing of `flavors.yaml`

[yeoldegrove]

What would you like to be added:

Add the git checkout of a certain commit and parsing of flavors.yaml to a public function, so that stuff like:
gardenlinux/glrd@f911488#diff-5bb0eb6209981098aa46d877fc4006c0c0242cb0c46551c3b4a72a86e7d360cdR171-R193 gets obsolete.

python-gardenlinux-lib/src/gardenlinux/flavors/__main__.py

Line 143 in a4c3480

flavors_data = _get_flavors_file_data(Path(Git().root, "flavors.yaml"))

could also use the new function to support parsing a commit.

Why is this needed:

This is a feature that is used frequently.