refactor(forks): Make Fork Pydantic-Compatible

[marioevz]

🗒️ Description

Modifies Fork type definition to make it pydantic-friendly, and can now be serialized (into a string) and parsed back into the correct object:

class BlockchainTest(BaseModel):
    fork: Fork = Field(..., alias="network")

instead of:

class BlockchainTest(BaseModel):
    fork: str = Field(..., alias="network")

And the conversion happens on the fly in both directions everywhere.

🔗 Related Issues

None.

✅ 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.

[danceratopz]

Really nice! Makes so much sense.

I just removed a one-line reference to blockchain_test_network_name in the docs.

Shame we can't quite get rid of the fixture class get_fork() method (due to potentially multi-fork state fixtures). We could apply some pydantic, if we strictly only supported single-fork state test fixtures, but it's nice to be able to maintain the ability to load multi-fork state test fixtures.