[Early Draft][Splicing] Partial, perform interactive transaction negotiation for new funding

[optout21]

This is a next step in the splicing partial implementation, including and depending on the following:

• [Splicing] Partial, handle splice_init & splice_ack messages #3407
• Refactor ChannelContext value fields into FundingScope #3592
• Refactor ChannelTransactionParameters into FundingScope #3604
• Dual funding extension: begin_interactive_funding_tx_construction #3443
• Extend begin_interactive_funding_tx_construction() with splicing-specific extra input (on top of #3443) #3516

Also relevant: #3418

Next step in splicing (#1621): This PR adds the implementation of performing the interactive transaction negotiation for the new post-splice funding transaction, after the splice_init & splice_ack handshake. Only the initiator can currently contribute.
The initiator adds the current funding tx as input, plus additional inputs, and the new funding as output plus a change output (if applicable).
After tx_complete the flow stops, does not yet follow to the commitment/signature exchange.

[optout21]

Rebased to current main, and current state of #3443 .

[optout21]

Rebased

[optout21]

Rebased; after #3513 many changes disappeared :)

[optout21]

Rebased, following channel phase #3418 . The channel manager part got much simpler!

[optout21]

Rebased up to 0216d7d #3629 from joostjager/log-attribution-failures Mar 4

Cannot rebase past that, df68774 #3016 somehow breaks my rebaseing...

[optout21]

Squashed some commits

[optout21]

Rebased on top of main (had to do magic with the commits...)

[optout21]

Extended implementation to handle last tx_complete, create signature on shared input, and emit FundingTransactionReadyForSigning event.

[codecov]

Codecov Report

Attention: Patch coverage is 77.64706% with 114 lines in your changes missing coverage. Please review.

Project coverage is 89.67%. Comparing base (c355ea4) to head (db27917).
Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/channel.rs	69.05%	107 Missing and 1 partial ⚠️
lightning/src/ln/channelmanager.rs	16.66%	5 Missing ⚠️
lightning/src/events/mod.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3444      +/-   ##
==========================================
+ Coverage   89.20%   89.67%   +0.46%     
==========================================
  Files         155      155              
  Lines      119377   124547    +5170     
  Branches   119377   124547    +5170     
==========================================
+ Hits       106496   111687    +5191     
+ Misses      10266    10227      -39     
- Partials     2615     2633      +18     

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[optout21]

Continued implementation: added funding_transaction_signed methods, tx_complete is handled on both sides.