[Splicing] Update Splicing msgs

[optout21]

Some minor updates to the spicing messages, to reflect the spec changes (from lightning/bolts#863 to lightning/bolts#1160 )
This is in preparation for splicing ( #1621 )
Prerequisites: None

Changes can be seen in #3274 , changes to file msgs.rs.

Changes include:

• some renames (splice -> splice_init, some field names)
• batched commitments support (commitment_signed)
• some new fields such as require_confirmed_inputs

[optout21]

My bad, this was already done with #3129 (it's both in main and in #3274, not sure why it shows there as diff ). Closing.