refactor(fw): re-write gas test generator as pytest plugin

[danceratopz]

🗒️ Description

Rewrites #719 as a pytest plugin; cf comments in #719 (review).

Still a WIP, to do:

• Handle data fixture.
• Refactor test_mcopy_huge_memory_expansion to use plugin
• oog fails.
• check plugin doesn't affect tests not marked with @ptyest.mark.generate_gas_test
• add testing
• clean-up how the data fixture is handled in the plugin;
• nice error message if data fixture is not available and with_data keyword is used. and vice versa?).
• nice error message if exact_gas_cost fixture is not available (cc @spencer-tb)

Fixtures seem to match, although test ids differ.

I've been executing fill as:

fill --fork=Cancun tests/cancun/eip5656_mcopy/test_mcopy_memory_expansion.py -m state_test -v -x --output=fixtures-after -s

🔗 Related Issues

#719

✅ 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: A PR with removal of converted JSON/YML tests from ethereum/tests have been opened.
• 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.