feat(flagd): use json schema validation instead of custom validation

[aepfli]

changed the code to rely on json schema instead of custom validation, should be more error proof for the future

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.47%. Comparing base (72e6bd3) to head (4d36628).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #282      +/-   ##
==========================================
- Coverage   94.21%   90.47%   -3.74%     
==========================================
  Files          20        5      -15     
  Lines        1020      189     -831     
==========================================
- Hits          961      171     -790     
+ Misses         59       18      -41     

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[gruebel]

Nice work

[aepfli]

Basically it is a good change, but i see a lot of problems currently, with inconsistencies with other languages - and i think we should refine this a little bit better, before merging this.