Update protocol generator to include definitions of performatives and fields

[solarw]

At the moment Message class generated does not have a good programmatic definition of performatives and fields relation

This relation check is done in code as

 Check correct contents
            actual_nb_of_contents = len(self._body) - DEFAULT_BODY_SIZE
            expected_nb_of_contents = 0
            if self.performative == SigningMessage.Performative.SIGN_TRANSACTION:
                expected_nb_of_contents = 2
                enforce(
                    isinstance(self.terms, CustomTerms),
                    "Invalid type for content 'terms'. Expected 'Terms'. Found '{}'.".format(
                        type(self.terms)
                    ),
                )
                enforce(
                    isinstance(self.raw_transaction, CustomRawTransaction),
                    "Invalid type for content 'raw_transaction'. Expected 'RawTransaction'. Found '{}'.".format(
                        type(self.raw_transaction)
                    ),
                )
            elif self.performative == SigningMessage.Performative.SIGN_MESSAGE:
                expected_nb_of_contents = 2
                enforce(
                    isinstance(self.terms, CustomTerms),
                    "Invalid type for content 'terms'. Expected 'Terms'. Found '{}'.".format(
                        type(self.terms)
                    ),
                )
                enforce(
                    isinstance(self.raw_message, CustomRawMessage),
                    "Invalid type for content 'raw_message'. Expected 'RawMessage'. Found '{}'.".format(
                        type(self.raw_message)
                    ),
                )

but can be replaced with something like

class SomeMessage:

PERFORMATIVES_FIELDS = {Performative1: (("field1", Field1Type), ("str_field", str))}

and all the checks can be done using this definition.

it will give more flexibility, like check on message creation with list of missing fields in error
introspection features, like code can check how to construct message or provide list of expected fields.

also can help to build automated tests of message encoding/decoding