feat: add `init` script to initialize a working devenv with a specific Home Assistant version

[palazzem]

Related Issues

• Closes #124

Proposed Changes:

This change ensures fixtures are aligned with the installed Home Assistant version to perfectly reproduce the core project testing environment. Specifically:

• Previous fixtures are removed as we don't need to maintain them anymore
• Added download_fixtures.sh script that downloads all fixtures and tests helpers for a given Home Assistant version
• Added init script that gather together all steps in a single command
• Documentation is updated to retrieve automatically the latest version of Home Assistant via GitHub API

Testing:

Follow the README.md instructions to initialize the devenv.

Extra Notes (optional):

n/a

Checklist

• [x] Related issues and proposed changes are filled
• [x] Tests are defining the correct and expected behavior
• [x] Code is well-documented via docstrings

[coveralls]

Pull Request Test Coverage Report for Build 7741554985

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 95.095%

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Totals
Change from base Build 7276681670:	0.0%
Covered Lines:	597
Relevant Lines:	629

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💛 - Coveralls