Mint: add Nostr Wallet Connect as a backend

[gudnuf]

This PR allows you to back your mint with any lightning wallet that supports Nostr Wallet Connect.

Adds:

• mint_nwc_url to settings
• NWCClient to nostr module
• nwc as lightning backend
• NwcRequestEvent and event kinds to event.py
• check if type is NWCRequestEvent in sign_event

[gudnuf]

Just realized there should be a get_info request to make sure the connected NWC supports all methods we need. Will add soon

[callebtc]

Really nice work, especially working it into the existing Python nostr client. Thank you.

[codecov]

Codecov Report

Attention: Patch coverage is 41.69611% with 165 lines in your changes missing coverage. Please review.

Project coverage is 65.30%. Comparing base (8675745) to head (d95eb4f).

Files	Patch %	Lines
cashu/nostr/nwc.py	46.73%	98 Missing ⚠️
cashu/lightning/nwc.py	26.31%	56 Missing ⚠️
cashu/nostr/event.py	45.00%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #565      +/-   ##
==========================================
- Coverage   69.02%   65.30%   -3.72%     
==========================================
  Files          92       94       +2     
  Lines        8674     8956     +282     
==========================================
- Hits         5987     5849     -138     
- Misses       2687     3107     +420     

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