pytest: add latest fork parameter and exit gracefully upon an invalid fork

[danceratopz]

Requires #1.

This PR builds upon #1. In addition, it:

1. Adds a custom exception InvalidForkError to ethereum_test_forks which is used to exit a pytest session gracefully if an invalid fork name is supplied to --latest-fork.
2. Adds an additional line to the pytest header displayed in the console output to help inform users that they're running with an (unstable) upcoming fork, or not. Hopefully, this will help avoid confusions such as in EIP 6780 Tests ethereum/execution-spec-tests#122 (comment) (I've struggled with this myself).

About 2. If --latest-fork is not specified, then the line:

Only executing fillers with stable/deployed forks: Specify an upcoming fork via --latest-fork=fork to run experimental fillers.

is displayed in the header.

Otherwise, if --latest-fork is specified:

Executing fillers up to and including FORK.

is displayed.

[danceratopz]

We could isolate this functionality in it's own small pytest plugin, if we think that our conftest.py will get too unwieldy.

[danceratopz]

I refactored the latest-fork functionality to be it's own pytest plugin (located in src/pytest_plugins/latest_fork.py). I think this helps keep the code modular and keep the pytest config in conftest.py manageable.