Add contentMediaType, contentEncoding and examples as a NonValidationKeyword

[jonnybbb]

contentMediaType, contentEncoding and examples should be treated as keywords according to the spec
https://json-schema.org/draft/2019-09/json-schema-validation.html#rfc.section.8.6 and https://json-schema.org/draft/2019-09/json-schema-validation.html#rfc.section.9.5

[codecov-io]

Codecov Report

Merging #341 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #341   +/-   ##
=========================================
  Coverage     69.85%   69.85%           
  Complexity      694      694           
=========================================
  Files            80       80           
  Lines          2737     2737           
  Branches        567      567           
=========================================
  Hits           1912     1912           
  Misses          612      612           
  Partials        213      213           

Impacted Files	Coverage Δ	Complexity Δ
...main/java/com/networknt/schema/JsonMetaSchema.java	85.82% <ø> (ø)	16.00 <0.00> (ø)

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[stevehu]

@jonnybbb Thanks a lof for your help.