[RFC] events: add InvoiceSent event for BOLT12 payee

[vincenzopalazzo]

When processing BOLT12 payments, there was an asymmetry in the API where payer nodes receive InvoiceReceived events but payee nodes had no way to access the invoices they created and sent as far I know.

This commit adds:

• New Event::InvoiceSent event generated when a BOLT12 invoice is created and sent in response to an invoice request
• New notify_bolt12_invoice_sent configuration parameter in UserConfig (defaults to false) to enable/disable the event generation

The InvoiceSent event provides the payee with access to:

• The created BOLT12 invoice
• The context of the BlindedMessagePath
• The payment hash and payment secret

[ldk-reviews-bot]

I've assigned @jkczyz as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 27 lines in your changes missing coverage. Please review.

Project coverage is 89.88%. Comparing base (217a5b0) to head (ee2f11c).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/events/mod.rs	0.00%	18 Missing ⚠️
lightning/src/ln/channelmanager.rs	43.75%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3853      +/-   ##
==========================================
- Coverage   89.93%   89.88%   -0.06%     
==========================================
  Files         163      163              
  Lines      131276   131307      +31     
  Branches   131276   131307      +31     
==========================================
- Hits       118062   118022      -40     
- Misses      10529    10600      +71     
  Partials     2685     2685              

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[TheBlueMatt]

Rather than doing this via the Event pipeline, we should figure out how to do it via the Flow that we added recently, I think.

When we first got the request to manually intercept payments between receiving the Invoice and sending the payment (for fedi), we added it via events, which was okay, but we tend to lean on the event pipeline too much generally and specifically for offers we have other flexibility concerns. Instead, we are trying to push things through the new flow, which can be configured separately.

I haven't had a chance to look at it yet, but #3833 seems to be trying to accomplish the same thing but via the new pipeline.

[vincenzopalazzo]

Thanks for looking into it! I am looking into #3833 now! Closing it for now!