Add Splicing message definitions

[optout21]

Splicing messages

• are defined in this BOLT PR (draft): Splice draft (feature 62/63) lightning/bolts#863
• a similar LDK PR, for dual-funded channel messages, is here (merged): Dual funding and interactive tx construction wire messages lightningdevkit/rust-lightning#1794 Dual funding and interactive tx construction wire messages lightningdevkit/rust-lightning#1794
• tracking issue is here: Dual-funded channels and Splicing Project Tracking lightningdevkit/rust-lightning#1621

The following messages should be added:
Splicing:

• splice (74)
• splice_ack (76)
• splice_locked (77)

Quiescence -- is this in this scope?:

• stfu (2)

The interactive transaction negotiation messages are already present (see PR mentioned above).

Note: the simplified proto in this repo (https://github.com/optout21/rust-lightning/tree/splicing-hapa1) also has splice_signed, but that is covered by the interaction tx negotiation messages.

[optout21]

Note: splice has type 74 in lightning/bolts#863 , but txabort also has 74 in lightning/bolts#851 .
Using value 84 for now.

[optout21]

Housekeeping: was added upstream long time ago (via lightningdevkit#2544 )