[sdk/python] Transfer transaction - support for message types

[cryptoBeliever]

Current Behaviour

Old SDKs message specification defined four types of messages:

• Plain Message (prefix 0)
• Encrypted Message (prefix 1)
• Persistent Delegated Message transaction (prefix 254)
• Raw Message (no prefix or not recognized prefix)

Currently, the SDK user (developer) has only the option to put the message as raw text when creating the transfer transaction object. So SDK user has to:

• remember (when sending and when reading from API transactions) which prefix is for which transaction type and how to decode messages
• implement encryption and decryption of messages (in case of encrypted messages)
• implement persistent delegating message creation

Expected Behaviour

It's desired that SDK users be released from the obligation to remember the specification for various types of messages.
For SDK developers would be very helpful if:

• he could create objects representing a specific message type (constructor/factory methods)
• when reading transactions from API provide a way to read messages without knowing the specification e.g. wrapping messages using a typed message object.

Corresponding issue for JS SDK: #228

[gimre-xymcity]

this is currently done via MessageEncoder object,
however, currently in case of NEM it will throw an error if it's instantiated on non-encrypted message type.
additionally try_decode/tryDecocde might throw in some cases

[cryptoBeliever]

Encrypting and decrypting message work correctly ✔️

encrypted = encoder.encode_deprecated(recipient_public_key, b'Some encrypted message')
print(encrypted.message)

# Create a transfer transaction
transfer = facade.transaction_factory.create({
	'type': 'transfer_transaction_v2',
	'signer_public_key': alicePubkey,
	'recipient_address': facade.network.public_key_to_address(bobKeyPair.public_key),
	'timestamp': networkTime.timestamp,
	'deadline': deadline,
	'fee': 200000,
	'amount': 1000000,
	'message': {
		'message_type': 'encrypted',
		'message': encrypted.message
	}
})

recipient_decoder = MessageEncoder(bobKeyPair)
decoded_message = recipient_decoder.try_decode(alicePubkey, encrypted)
print(decoded_message)

encrypted_message = message_encoder.encode(bobPubkey, b'message')
print(encrypted_message)

# Create a transfer transaction
transfer = facade.transaction_factory.create({
	'type': 'transfer_transaction_v1',
	'signer_public_key': alicePubkey,
	'recipient_address': facade.network.public_key_to_address(bobPubkey),
	'deadline': deadline,
	'fee': 100000,
	'mosaics': [],
	'message': b'\1' + bytes(encrypted_message[1::].hex(), 'utf-8') # workaround for old wallet (there is bug in old SDK that causes double encoding)
})