feat(forks): Implement `transition_time` method

Should we make the `15_000` a constant based of the `fork`. We might need to add this in the `forks.py` logic.
Maybe a function from `class Paris` (and in base forks) called `transition_time()` that returns `15_000`. All the other forks (incl. Prague) would inherit this.

We could then use `fork.transition_time() + 1` etc

_Originally posted by @spencer-tb in https://github.com/ethereum/execution-spec-tests/pull/1394#discussion_r2046450583_