[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]

Codecov Report

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

Project coverage is 89.62%. Comparing base (02973ea) to head (8cfd5d9).
Report is 13 commits behind head on main.

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

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3316      +/-   ##
==========================================
- Coverage   89.65%   89.62%   -0.03%     
==========================================
  Files         126      126              
  Lines      102532   102763     +231     
  Branches   102532   102763     +231     
==========================================
+ Hits        91922    92104     +182     
- Misses       7887     7936      +49     
  Partials     2723     2723              

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