Add gson annotations for fields and getter/setters

[dotasek]

At present only Jackson is supported as a JSON serializer/deserializer. This adds annotations that will allow gson to also be used.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 12.76%. Comparing base (717ef9c) to head (cf1456a).
Report is 30 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1824   +/-   ##
=========================================
  Coverage     12.76%   12.76%           
- Complexity    33138    33139    +1     
=========================================
  Files          2221     2221           
  Lines        676651   676669   +18     
  Branches     199414   199419    +5     
=========================================
+ Hits          86393    86403   +10     
- Misses       559613   559614    +1     
- Partials      30645    30652    +7     

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[dotasek]

This is reviewed in a downstream project here: hapifhir/org.hl7.fhir.validator-wrapper#195 (review)