[Splicing] Partly handle splice_init & splice_ack messages

[optout21]

Handle splice_init & splice_ack messages, without performing splicing. splice_init does not error, but splice_ack returns an error (and neither side continues with splicing). One next piece for splicing #3298 . This can come before #3137 .

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.

Project coverage is 89.23%. Comparing base (0c31021) to head (fb4968c). Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/channel.rs	80.00%	2 Missing :warning:
lightning/src/ln/channelmanager.rs	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3407 +/- ## ========================================== + Coverage 89.22% 89.23% +0.01% ========================================== Files 130 130 Lines 106965 106969 +4 Branches 106965 106969 +4 ========================================== + Hits 95438 95459 +21 + Misses 8734 8720 -14 + Partials 2793 2790 -3 ```

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[optout21]

I have reorganized code, moved most single-channel checks and handling logic into Channel struct from ChannelManager.

[optout21]

Rebased (post #3137) and squashed