Ensure ability to use selector fn override

[antazoey]

Overview

I tried to use the selector_fn override but struggled in the case of Starknet which uses longer, int-based selectors.
I think the problem is with the int type so my suggestion for ethpm-types is to make a good test case of switching the selector fn to Starknet style and using it. I know work needs to be done here but not 100% sure what it is but making this ticket as a reminder to do it!

Specification

class CustomContractType(ContracType):
  _selector_fn = <some-int-hash-based-look-up>

def test_custom_int_selectors(contract_dict):
   contract_type = ContractType.parse_obj(contract_dict)
   actual = contract_type.mutable_methods[32451345]
   expected = contract_type.MyExpectedMethod
   assert actual == expected

Dependencies

Include links to any open issues that must be resolved before this feature can be implemented.