T3225
As per feedback from D14417, this diff forms part of a series of diffs to incrementally implement the key concepts from D14417:
The coinselection and txFee logic takes into consideration both the extra OP_RETURN outputs and the byte size of each OP_RETURN script.
```
1. Get a working API for N x p2pkh outputs
2. Extend with N x op_return **(this diff)**
3. Extend with N x slp outputs
4. Extend with N x p2sh outputs
```