feat: allow self payment

[shruti2522]

fix #2462

• implemented the following in outbound_payment.rs :

  • if the router tells us the payment is to ourselves, we build a simple one-hop route and treat it like a local payment
  • when preimage is available, immediately claim the HTLC

• added test case self_spontaneous_payment_fulfills() to test self payment

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[TheBlueMatt]

This violates a number of the API constraints on sending payments - specifically, someone could send a payment, then restart and they wouldn't see the pending payment, thinking they hadn't "sent" it. Check out some of the early feedback on the first attempt at this #2573

[codecov]

Codecov Report

❌ Patch coverage is 95.71429% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.54%. Comparing base (e9ce486) to head (845b3ab).

Files with missing lines	Patch %	Lines
lightning/src/ln/outbound_payment.rs	95.71%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4328      +/-   ##
==========================================
- Coverage   86.54%   86.54%   -0.01%     
==========================================
  Files         158      158              
  Lines      103166   103232      +66     
  Branches   103166   103232      +66     
==========================================
+ Hits        89287    89338      +51     
- Misses      11456    11469      +13     
- Partials     2423     2425       +2     

Flag	Coverage Δ
fuzzing	37.03% <34.88%> (-0.04%)	⬇️
tests	85.83% <95.71%> (-0.01%)	⬇️

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