[Splicing] Clone for ChannelContext

[optout21]

Update: This assumes that the whole ChannelContext is duplicated, which may not be the case in the final solution.

Cloning of a channel -- more precisely of ChannelContext -- is needed for Splicing as well as for dual funding RBF, this is a preparation.

Simple #[derive(Clone)) is not sufficient, as the channel signer struct cannot be cloned, ultimately due to AtomicUSize and Secp256k1 (in KeysManager). So instead, a field-by-field cloning is done, with the a few exceptions.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.65%. Comparing base (66fb520) to head (e593e86).
Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3332      +/-   ##
==========================================
- Coverage   89.66%   89.65%   -0.01%     
==========================================
  Files         126      126              
  Lines      102676   102826     +150     
  Branches   102676   102826     +150     
==========================================
+ Hits        92062    92188     +126     
- Misses       7894     7913      +19     
- Partials     2720     2725       +5     

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	89.65% <100.00%> (+0.01%)	⬆️

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[jkczyz]

Discussed briefly offline. This may not be needed if we split ChannelContext to avoid duplicating some fields.

[optout21]

This change request assumes that the whole ChannelContext struct is duplicated, which may not be the best solution. I put this on hold for now (back to draft, description updated).