Allow user-defined field types to implement their own attribute function

When using user-defined fields ( such as EnumField as requested in #506 or Union from marshmallow-union) it would be nice to be able to hook directly into apispec.ext.marshmallow.field_converter.FieldConverterMixin.field2property by defining a specific method.

I'm currently using this simple code for that purpose:

def customfield2properties(self: OpenAPIConverter, field: fields.Field, ret: Dict[str, Any]) -> Dict[str, Any]:
    """
    A field attribute function to allow field classes to declare
    how to generate/add parameters to their jsonschema representation
    """
    if hasattr(field, "apispec_field2properties"):
        field.apispec_field2properties(self, ret)
    return ret

Once added to the converter with any field defining the apispec_field2properties method can describe itself without having to add a specific attribute_function to the Marshmallow plugin.

Is this something you would be interested in having in apispec? I can work on a pull request.

Start at apispec.ext.marshmallow.field_converter.FieldConverterMixin.field2property and compare it with the proposed customfield2properties example. Determine how user-defined fields can supply their own JSON Schema attributes without registering a Marshmallow plugin attribute function. Done means the hook is supported and its behavior is covered by tests.