Make Money serializable

[leonardehrenfried]

Summary

@fpurcell has reported (and I have independently also noticed) that when enabling Fares V2 Money objects fail serialization because they are not Serializable.

This PR fixes that.

Unit tests

I'd love to write one but I'm not sure if it's worth it. Test added.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (9695a96) 66.59% compared to head (faed0c2) 66.59%.
Report is 5 commits behind head on dev-2.x.

Additional details and impacted files

@@            Coverage Diff             @@
##             dev-2.x    #5419   +/-   ##
==========================================
  Coverage      66.59%   66.59%           
- Complexity     15284    15286    +2     
==========================================
  Files           1790     1790           
  Lines          69388    69388           
  Branches        7308     7308           
==========================================
+ Hits           46206    46207    +1     
  Misses         20703    20703           
+ Partials        2479     2478    -1     

Files	Coverage Δ
...org/opentripplanner/transit/model/basic/Money.java	76.00% <ø> (ø)

... and 1 file with indirect coverage changes

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