feat: ability to get conversion classes by name

[antazoey]

What I did

This helps me access the ENS conversion class, which makes more sense than any of these, but is still helpful.

This is what I have to do in the meantime to get access to ENS in the ape-ens tests:

@pytest.fixture(scope="session")
def converter(chain):
    # TODO: Replace with https://github.com/ApeWorX/ape/pull/2497
    #   (requires ape 0.8.26).
    converters = chain.conversion_manager._converters[AddressType]
    for converter in converters:
        if isinstance(converter, ENSConversions):
            return converter

    pytest.fail("Failed to find ENS converter registered in Ape.")

Ape should be good about testing stuff like ape-ens

How I did it

How to verify it

Checklist

• All changes are completed
• Change is covered in tests
• Documentation is complete

[fubuloubu]

I think in some of the other plugins I used package name (minus ape-)

[antazoey]

that may not work the best here, as ape-ethereum includes the WeiConversions etc.