[Feature Request] Add option to withdraw command for omitting broadcast of transaction

[1ma]

Often when I close a channel and had little liquidity on my side I do a LN swap-in with that small UTXO.

However services such as Boltz and similar require to send an UTXO of an exact value. And withdraw's options, while already very powerful, sometimes produce a TX with an UTXO 1 sat larger or smaller than I would expect, I guess due to rounding vBytes. To avoid this issue I do a first hop to a more feature-complete wallet (Sparrow) where I can see what the final transaction will look like before I broadcast it.

This extra hop friction would be solved with an optional flag to withdraw that prompts it to omit the broadcasting of the transaction, but still shows its hex serialization on stdout. This way one could load the finalized transaction on a visualization tool such as Sparrow Wallet, check that the UTXO value matches exactly what the atomic swap expects, and prepare a new transaction or atomic swap if it doesn't. When it does, you'd broadcast the transaction manually.