refactor(fw): augment & use Fixtures model in fixture collection

[danceratopz]

🗒️ Description

This is a gentle refactor to make use of Fixtures during fixture collection and writing:

• Moves collect_info_file() from the BaseFixture class to Fixtures. It now also takes a file instead of an fd for consistency with other Fixtures methods.
• Use Fixtures model in the FixtureCollector class.

🔗 Related Issues

Makes use of #501.

✅ Checklist

• All: Set appropriate labels for the changes.
• All: Considered squashing commits to improve commit history.
• All: Added an entry to CHANGELOG.md.
• All: Considered updating the online docs in the ./docs/ directory.
• Tests: All converted JSON/YML tests from ethereum/tests have been added to converted-ethereum-tests.txt.
• Tests: Included the type and version of evm t8n tool used to locally execute test cases: e.g., ref with commit hash or geth 1.13.1-stable-3f40e65.
• Tests: Ran mkdocs serve locally and verified the auto-generated docs for new tests in the Test Case Reference are correctly formatted.

[marioevz]

LGTM!