Make prevtx in TxAddInput message an Option

[dunxen]

It looks like there are cases where prevtx can be omitted.
See recent spec meeting notes here: lightning/bolts#1129 (comment) and the linked Delving Bitcoin post by t-bast, specifically:

That means that for lightning, we should only drop the prevtx field if:

• this is a splice transaction of a taproot channel (there is a shared input that uses taproot and requires signatures from both participants)
• or each participant adds at least one taproot input to the transaction

[wpaulino]

Looks like this is being pushed back for now: niftynei/lightning-rfc#18 (comment)

[dunxen]

Looks like this is being pushed back for now

Thanks. It makes sense to close this as the comment you linked to imply additional work beyond the original suggestion.