feat: Add Unit Specific Limits

[cloudsupper]

This PR introduces functionality to set unit-specific limits for:

• PEG_INS
• PEG_OUTS
• MAX_BALANCE

[callebtc]

This is awesome thank you. Can we use the opportunity to rename these flags to "max mint" and "max melt" instead of peg-in and peg-out? this is old baggage that I wanted to get rid of for a while

[callebtc]

Calling crud.get_balance should be discouraged because the view takes a very long time to query on sqlite if the database gets moderately large. Instead, since https://github.com/cashubtc/nutshell/pull/705 there is a balance for each keyset that is being tracked which should be pretty much instant to get.

[codecov[bot]]

Codecov Report

:x: Patch coverage is 71.05263% with 22 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 60.78%. Comparing base (15ec97b) to head (9fc26b6).

Files with missing lines	Patch %	Lines
cashu/mint/features.py	8.33%	11 Missing :warning:
cashu/mint/verification.py	81.96%	11 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #740      +/-   ##
==========================================
- Coverage   61.10%   60.78%   -0.32%     
==========================================
  Files          89       89              
  Lines       10565    10622      +57     
==========================================
+ Hits         6456     6457       +1     
- Misses       4109     4165      +56     

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