Add OpenAPI JsonMetaSchema

[justin-tay]

This adds the OpenAPI JsonMetaSchema and associated vocabulary.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.65217% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 79.07%. Comparing base (48ca3c2) to head (36b2848).

Files	Patch %	Lines
.../main/java/com/networknt/schema/oas/OpenApi30.java	88.88%	1 Missing ⚠️
.../main/java/com/networknt/schema/oas/OpenApi31.java	95.45%	1 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1011      +/-   ##
============================================
+ Coverage     78.90%   79.07%   +0.17%     
- Complexity     1965     1973       +8     
============================================
  Files           172      174       +2     
  Lines          6352     6395      +43     
  Branches       1255     1258       +3     
============================================
+ Hits           5012     5057      +45     
+ Misses          867      866       -1     
+ Partials        473      472       -1     

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