[sdk/python] NEM - automatic population for fields levy_size and message_envelope_size

[cryptoBeliever]

Current Behaviour

SDK user has to specify fields manually:

• levy_size
• message_envelope_size

Expected Behaviour

Fields levy_size and message_envelope_size can be calculated based on other values in the transaction. We don't need to force users to calculate and specify it manually.

The corresponding issue for Javascript SDK: #293

[cryptoBeliever]

I've tested Python SDK on the current dev branch, and SDK users don't need any more fill:

• message_envelope_size when sensing transfer transaction
• levy_size when sending mosaic definition transaction

I've not noticed any issues 👍

Example tx construction for transfer transaction:

transfer = facade.transaction_factory.create({
	'type': 'transfer_transaction_v2',
	'signer_public_key': alicePubkey,
	'recipient_address': bobAddress,
	'timestamp': networkTime.timestamp,
	'deadline': deadline,
	'fee': 100000,
	'amount': 1000000,
	'message': {
		'message_type': 'plain',
		'message': 'Hello Bob! Here is 1 XEM!'
	}
})

Example ex construction for mosaic creation with levy:

transfer = facade.transaction_factory.create({
	'type': 'mosaic_definition_transaction_v1',
	'signer_public_key': alicePubkey,
	'timestamp': networkTime.timestamp,
	'deadline': deadline,
	'rental_fee': 10000000,
	'fee': 200000,
	'rental_fee_sink': 'TBMOSAICOD4F54EE5CDMR23CCBGOAM2XSJBR5OLC',
	'mosaic_definition': {
		'owner_public_key': alicePubkey,
		'id': {'namespace_id': {'name': b'supernamespace'}, 'name': b'awesome_mosaic1'},
		'description': b'Just testing',
		'properties': [
			{'property_': {'name': b'divisibility', 'value': b'0'}},
			{'property_': {'name': b'initialSupply', 'value': b'1000'}},
			{'property_': {'name': b'supplyMutable', 'value': b'true'}},
			{'property_': {'name': b'transferable', 'value': b'true'}}
		],
		'levy': {
			'transfer_fee_type': 'absolute',
			'recipient_address': aliceAddress,
			'mosaic_id': {
				'namespace_id': {'name': b'nem'},
				'name': b'xem',
			},
			'fee': 0x00000000037706A6
		}
	},
})