CLI: split and combine e-cash

[elsirion]

Adds two functions that can be used to manipulate serialized e-cash bags.

[codecov]

Codecov Report

Attention: 48 lines in your changes are missing coverage. Please review.

Comparison is base (0cf565b) 57.13% compared to head (b810ad6) 57.07%.

Files	Patch %	Lines
fedimint-cli/src/client.rs	0.00%	45 Missing ⚠️
fedimint-core/src/config.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3879      +/-   ##
==========================================
- Coverage   57.13%   57.07%   -0.06%     
==========================================
  Files         193      193              
  Lines       42608    42656      +48     
==========================================
+ Hits        24344    24346       +2     
- Misses      18264    18310      +46     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.