Basic Splicing

[optout21]

Basic implementation of channel splicing, with limitations (tracking: #1621).
Support channel splicing between two LDK instances, increasing channel capacity by adding some UTXOs, resulting in a usable channel.
Includes support for following messages: splice_init, splice_ack, tx_add_input, tx_add_output, tx_complete, tx_signatures, splice_locked.

The following limitations are acceptable, and handled later (it's always fine to include more right away):

• No quiescence is used/checked
• Only splice-in supported, splice-out not
• Splice from V2 channel only (dual funded), not from V1 channel
• Acceptor does not contribute inputs to the splice
• RBF not supported
• between two LDK instances
• Interruptions and restarts may not be handled
• Defer some complex error scenarios

Some prerequisites:
#3137
#2302
#3294
#3295