[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:

• #3407
• #3592
• #3604
• #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 0216d7dfc27ea758f8d9a053d559b7fa096e4322 #3629 from joostjager/log-attribution-failures Mar 4

Cannot rebase past that, df6877445e3c3d05d772899bf12b9fe6178d08fd #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[bot]]

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 :warning:
lightning/src/ln/channelmanager.rs	16.66%	5 Missing :warning:
lightning/src/events/mod.rs	0.00%	1 Missing :warning:

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.

[optout21]

Closing as this prototype is no longer valid/needed 😎