Extract FinishTransaction from send()
Extract FinishTransaction from send()

The final step of send either produces a PSBT or the final transaction.
We extract these steps to a new helper function `FinishTransaction()` to
reuse them in `sendall`.

Github-Pull: #24118
Rebased-From: 902793c7772e5bdd5aae5b0d20a32c02a1a6dc7c

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