lightning-invoice: Add Description::as_inner

[MaxFangX]

v0.0.119 introduced a lightning_invoice::Description newtype which doesn't allow accessing the inner UntrustedString without cloning the entire Description. Adds a quick fn Description::as_inner -> &UntrustedString which users can use to get the underlying &str if needed.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 89.71%. Comparing base (951174a) to head (2a8ab6f).

Files	Patch %	Lines
lightning-invoice/src/lib.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3203      +/-   ##
==========================================
- Coverage   89.77%   89.71%   -0.06%     
==========================================
  Files         122      122              
  Lines      101495   101498       +3     
  Branches   101495   101498       +3     
==========================================
- Hits        91114    91064      -50     
- Misses       7693     7737      +44     
- Partials     2688     2697       +9     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[tnull]

LGTM