Add proper signet support for invoices

[devrandom]

No description provided.

[codecov]

Codecov Report

Merging #943 (bac5f49) into main (3996eaa) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #943      +/-   ##
==========================================
+ Coverage   90.59%   90.61%   +0.01%     
==========================================
  Files          60       60              
  Lines       30393    30397       +4     
==========================================
+ Hits        27536    27543       +7     
+ Misses       2857     2854       -3     

Impacted Files	Coverage Δ
lightning-invoice/src/lib.rs	88.88% <ø> (ø)
lightning-invoice/src/de.rs	80.86% <100.00%> (+0.07%)	⬆️
lightning-invoice/src/ser.rs	92.19% <100.00%> (+0.05%)	⬆️
lightning/src/ln/functional_tests.rs	97.22% <0.00%> (+0.04%)	⬆️

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