Add Splicing messages

[optout21]

As part of Splicing, add needed protocol messages.

The messages are defined in the LN spec PR 863 (draft).
Note that the PR is currently a draft, it needs a rebase, and does not reflect the latest state of discussions.

The following messages should be added:

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

The interactive TX negotiation messages (also used by Splicing) are already present. They are a good example for this task (PR #1794 ).
Note that the stfu quiescence message may be out of scope here currently.

[TheBlueMatt]

Like we did for channel-open-v2 I don't see anything wrong with landing this before we handle them, as long as we have some level of confidence that the spec isn't gonna change much going forward.