all: Abstract fixtures, StateTest refactor

[marioevz]

🗒️ Description

Accomplishes the first steps to implement a fully functional StateTest generator in execution-spec-tests:

• Abstracts Fixture into a BaseFixture, in order to be able to implement a new fixture type, that is StateTest
• Refactors the existing functionality of StateTest (i.e. generating a simple blockchain test) into a class that generates a BlockchainTests. This class should, in a follow-up PR, generate a StateTestFixture, depending on the parameters used to run the test filler.
• Refactored the spec folder to include a subfolder per test type (at the moment only blockchain and state)
• Moved types FixtureHeader, FixtureExecutionPayload, FixtureEngineNewPayload, FixtureBlock and Fixture into spec/blockchain/types.py, since these types are pertinent only to the BlockchainTest type.

At the moment, the functionality and output fixtures are (or should be) unchanged.

Remaining tasks (follow-up PRs):

• spec/state/types.py should have its own class Fixture that represents an output fixture of the state tests.
• Refactor the FixtureCollector class so it behaves differently depending on the test type that it's collecting (in my opinion, we should collect only one type per run, so it should panic if it finds two different types, but this is open to discussion)

🔗 Related Issues

Towards fixing #343

✅ 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: 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.