[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]

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 ⚠️
lightning/src/ln/channelmanager.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            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