feat: unique invoice listeners for extensions

[dni]

changes the register_invoice_listener to use a non unique key so you its not possible to have a listeners running twice and throw a warning when trying to.

we should discourage use of get_current_extension_name its not needed, easy enough for the extension dev to pass the extension name or whatever he wants his listener to be named.

@talvasconcelos some extension still don't use a name, but they should! can you check that? also they should not use get_current_extension_name() anymore

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (5264677) 59.31% compared to head (0329616) 59.29%.

Files	Patch %	Lines
lnbits/tasks.py	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #2250      +/-   ##
==========================================
- Coverage   59.31%   59.29%   -0.02%     
==========================================
  Files          60       60              
  Lines        9041     9044       +3     
==========================================
  Hits         5363     5363              
- Misses       3678     3681       +3     

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