[Splicing] Signer extended with method to sign prev funding transaction input

[optout21]

Fixes #3312 . (#1621 )

New method to sign an input of a transaction with our funding key. Used for splicing, when signing the previous funding transaction. The previous funding transaction becomes an input to the new funding transaction, and it is a multisig, which we also need to sign.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 26 lines in your changes missing coverage. Please review.

Project coverage is 89.56%. Comparing base (0db051b) to head (f22b011). Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/sign/mod.rs	0.00%	20 Missing :warning:
lightning/src/util/test_channel_signer.rs	0.00%	6 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3316      +/-   ##
==========================================
- Coverage   89.61%   89.56%   -0.05%     
==========================================
  Files         127      127              
  Lines      103534   103560      +26     
  Branches   103534   103560      +26     
==========================================
- Hits        92781    92756      -25     
- Misses       8053     8101      +48     
- Partials     2700     2703       +3     

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[optout21]

Documentation updated per the suggestions

[optout21]

LGTM modulo one small change

Changed (also rebased)