Bug fixes and improvements in serialization/deserialization

[hailin-lee]

• Rewrite enums with additional attributes to remove enum_field with custom encoder/decoder
  • Override __new__ to assign additional attributes to enum members
  • Fix bug that previous enum encoder/decoder do not handle None value
• Handle None value when encoding/decoding datetime fields
• Omit fields with None value in JSON serialization

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.26%. Comparing base (43fc113) to head (1a0bf87).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #52   +/-   ##
=======================================
  Coverage   96.26%   96.26%           
=======================================
  Files          46       46           
  Lines        1044     1044           
=======================================
  Hits         1005     1005           
  Misses         39       39           

Flag	Coverage Δ
pytest	96.26% <0.00%> (ø)

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