feat(forks): Implement `transition_time` method

[marioevz]

          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 #1394 (comment)