Add tx_signatures.tlvs field (splicing-specific field in dual funding message)

[optout21]

Two relatively small changes, in preparation for splicing:

• New tx_signatures.tlvs field. This field is splicing-specific, and is used for exchanging signature on the previous funding tx, which is a shared input. The spec of this is still non-finalized, just like for the rest of the already-added splicing messages (spec PRs Add funding outpoint sigs to tx_signatures message lightning/bolts#1009 Splice draft (feature 62/63) lightning/bolts#863)

• Place all Splicing messages -- which are still only placeholders without real functionality -- behind the recently-introduced dual_funding feature flag. Splicing relies on dual funding, and is not usable without dual funding.

Note: command for testing:

RUSTFLAGS="--cfg=dual_funding" cargo test

[TheBlueMatt]

LGTM, modulo @dunxen's comments.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 70.00000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 89.66%. Comparing base (1d2a27d) to head (bdbb177).
Report is 17 commits behind head on main.

Files	Patch %	Lines
lightning/src/ln/msgs.rs	70.00%	3 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2991      +/-   ##
==========================================
+ Coverage   89.40%   89.66%   +0.25%     
==========================================
  Files         117      117              
  Lines       96016    98874    +2858     
  Branches    96016    98874    +2858     
==========================================
+ Hits        85842    88653    +2811     
- Misses       7957     8018      +61     
+ Partials     2217     2203      -14     

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[optout21]

Renamed field to more descriptive as suggested.