Wallet: fix secret derivation & new CLI command cashu selfpay

[callebtc]

Removes the unnecessary hashing in the secret derivation.

Note: old tokens can't be restored after this upgrade. Self-payment is necessary to be able to restore all tokens again.

Use: new CLI command cashu selfpay to recycle tokens once.

[codecov]

Codecov Report

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

Comparison is base (0959314) 68.94% compared to head (7e60410) 69.23%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #331      +/-   ##
==========================================
+ Coverage   68.94%   69.23%   +0.28%     
==========================================
  Files          52       52              
  Lines        4193     4212      +19     
==========================================
+ Hits         2891     2916      +25     
+ Misses       1302     1296       -6     

Files	Coverage Δ
cashu/wallet/cli/cli_helpers.py	52.83% <100.00%> (+18.86%)	⬆️
cashu/wallet/secrets.py	91.35% <100.00%> (ø)
cashu/wallet/wallet.py	89.25% <ø> (ø)
cashu/wallet/cli/cli.py	58.63% <89.47%> (+1.30%)	⬆️

... and 1 file with indirect coverage changes

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