Message payload wrapper
### deserialize(serialised_data): [MessagePayload](#messagepayload)
Turns a serialized message payload string back into its class
| Parameter | Type | Description |
| --------------- | -------- | -------- |
| serialised_data | String | |
### payloadType(): [MessagePayloadType](#messagepayloadtype)
Get the type of message this contains (used to select the correct getter)
### asIndexation(): [IndexationPayload](#indexationpayload)
### asMilestone(): [MilestonePayload](#milestonepayload)
### asTransaction(): [TransactionPayload](#transactionpayload)
### asReceipt(): [ReceiptPayload](#receiptpayload)
### asTreasury(): [TreasuryPayload](#treasurypayload)
### serialize(): String
Serializes the message payload into a json string