refactor: improve multi payment (builder review)

[enthusiastmartin]

Builder review suggestions to use origin check rather than having a list of authorities which can add or remove accepted currency.

Also consolidated multi-payment config.

[codecov]

Codecov Report

Merging #170 (36daed1) into master (6725da2) will decrease coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #170      +/-   ##
==========================================
- Coverage   86.11%   85.99%   -0.13%     
==========================================
  Files          21       21              
  Lines        2125     2092      -33     
==========================================
- Hits         1830     1799      -31     
+ Misses        295      293       -2     

Impacted Files	Coverage Δ
.../transaction-multi-payment/benchmarking/src/lib.rs	100.00% <100.00%> (ø)
pallets/transaction-multi-payment/src/lib.rs	75.20% <100.00%> (-2.10%)	⬇️

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[mrq1911]

note: there is no migration that would kill whole authorities storage root, this shouldn't be a problem, because we have no authorities set yet anyway