Invoice utils: allow to set both description hash and payment hash

[tnull]

I discovered we had no API method to specify both the description hash and the payment hash at the same time.

Here, we add such a helper method, although mid-term we probably should move to a different pattern here (builder?) as the current way is rather inflexible and the method names get ridiculously long.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 27 lines in your changes missing coverage. Please review.

Project coverage is 89.59%. Comparing base (6004ee5) to head (43fd78b).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/invoice_utils.rs	0.00%	27 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3371      +/-   ##
==========================================
- Coverage   89.62%   89.59%   -0.04%     
==========================================
  Files         127      127              
  Lines      103534   103762     +228     
  Branches   103534   103762     +228     
==========================================
+ Hits        92797    92963     +166     
- Misses       8038     8105      +67     
+ Partials     2699     2694       -5     

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[jkczyz]

Can't recall if there was a reason why we can't take Bolt11InvoiceDescription as a parameter instead.

[tnull]

I assume the lifetimes don't lend themselves to being exposed in bindings?

[jkczyz]

Ah, right. Though an owned version would work, I suppose. No need to do it in this PR.

[TheBlueMatt]

We really need to replace this garbage with some kind of config struct/builder pattern so its not a total mess. #3375