feat: balance check as signed extension

[enthusiastmartin]

Added signed extension for multi-payment pallet to check account balance when set_currency call is executed.

Balance must > 0 atm.

[codecov]

Codecov Report

Merging #115 (187017c) into master (3526cd9) will decrease coverage by 0.21%.
The diff coverage is 62.50%.

@@            Coverage Diff             @@
##           master     #115      +/-   ##
==========================================
- Coverage   87.30%   87.08%   -0.22%     
==========================================
  Files          17       17              
  Lines        1796     1812      +16     
==========================================
+ Hits         1568     1578      +10     
- Misses        228      234       +6     

Impacted Files	Coverage Δ
pallets/transaction-multi-payment/src/lib.rs	78.01% <62.50%> (-1.99%)	⬇️

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