Reintroduce cfg(dual_funding) for handling of open_channel2 messages

[dunxen]

There is still some work to be done here (like #3423) and it probably makes sense to release this (around v0.2) with the ability to open V2 channels too. This would also make our functional tests for dual-funding more complete.

[codecov]

Codecov Report

Attention: Patch coverage is 70.58824% with 5 lines in your changes missing coverage. Please review.

Project coverage is 89.77%. Comparing base (6e85a0d) to head (e5f6e4e).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/channelmanager.rs	80.00%	3 Missing ⚠️
lightning/src/ln/peer_handler.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3485      +/-   ##
==========================================
+ Coverage   89.71%   89.77%   +0.06%     
==========================================
  Files         130      129       -1     
  Lines      107625   112231    +4606     
  Branches   107625   112231    +4606     
==========================================
+ Hits        96553   100760    +4207     
- Misses       8672     9166     +494     
+ Partials     2400     2305      -95     

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