better handle list types

[matthew-emw]

where our GraphQL schema has a list field as follows:

accounts(
    # ...
    """sort the rows by one or more columns"""
    order_by: [accounts_order_by!]
    # ...
  )

we were observing generated code with non-list arguments like:

def accounts(
        cls,
        *,
        # ...
        order_by: Optional[accounts_order_by] = None,
        # ...
    )

this is pehaps related to issue #321 and PR #327.

my PR reworks this to result in:

def accounts(
        cls,
        *,
        # ...
        order_by: Optional[List[accounts_order_by]] = None,
        # ...
    )

i'd like to extend my PR with tests, but:

• i see ArgumentGenerator is currently not covered by any tests
• in custom_operations.py i see both an ArgumentsGenerator being injected and an ArgumentGenerator (no ess) being instantiated
• does this imply a direction of travel and that ArgumentsGenerator should be used going forward?

i'd be grateful for any guidance. thanks!