Consider using type generics with the StacksTransactionWire type

[aryzing]

Problem

Different transaction types often require dedicated code. However, even after checking the payload type, the payload needs to be kept in its own variable to preserve any type checks or assertions. There is no way to assert that a transaction has a given payload type

Solution

Introduce generics for transaction types. Something like

type StacksTransactionWire<TPayload = PayloadWire> = {
  payload: TPayload
  // ... other props
}

This would allow typing a given transaction's payload (and possibly other internal structures) without having to keep them as separate variables.