Allow `dump_by` to take lambdas

[kunalbhagawati]

If we have a field that deserializes to an Enum but serializes from an object with an enum attribute, will it be easier to let the dump_by attribute to be callable?
This will avoid the hassle of creating a Method field with custom serialization and deserialization.

For example,
If the API request body is:

{
  "name": "Some user",
  "role": "PARENT" // enum here
}

and the view builds a user object, where the role is an sqlalchemy enum field (assuming role.label is an enum), then it would need to serialize from role.label.

Right now even subclassing wont work since __init__ raises a validation error:

class FromObjectEnumField(EnumField):
    def _serialize(self, value, attr, obj):
        if callable(self.dump_by):
            value = self.dump_by(value)
        return super()._serialize(value, attr, obj)

# raises `ValueError: Invalid selection for load_by must be EnumField.VALUE or EnumField.NAME, got <function UserSchema.<lambda> at 0x10c8ce040>` on `__init__`

Forcing me to override init as well. Is there a better way to handle this?

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marshmallow-enum==1.5.1