Support readOnly and writeOnly for required validation

[justin-tay]

Closes #1185

Add support for ignoring readOnly or writeOnly fields for required validation.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 72.22222% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.29%. Comparing base (48ca3c2) to head (dabc409).
⚠️ Report is 94 commits behind head on master.

Files with missing lines	Patch %	Lines
...n/java/com/networknt/schema/RequiredValidator.java	68.75%	1 Missing and 4 partials ⚠️
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Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1186      +/-   ##
============================================
- Coverage     78.90%   78.29%   -0.62%     
- Complexity     1965     2080     +115     
============================================
  Files           172      201      +29     
  Lines          6352     6873     +521     
  Branches       1255     1260       +5     
============================================
+ Hits           5012     5381     +369     
- Misses          867     1019     +152     
  Partials        473      473              

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