Allow customization of const message to include input value

[justin-tay]

Closes #846

This makes it possible for a custom locale properties file to generate the desired message for the const validator.

This doesn't make it the default as doing so will make it inconsistent with the rest of the messages for other validators.

The input node value that caused the failure can be consistently retrieved using message.getInstanceNode() and its path from message.getInstanceLocation() and a custom OutputFormat implementation can be used to consistently format the messages.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.74%. Comparing base (48ca3c2) to head (8b0b859).
Report is 21 commits behind head on master.

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Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1057      +/-   ##
============================================
- Coverage     78.90%   78.74%   -0.16%     
- Complexity     1965     2033      +68     
============================================
  Files           172      191      +19     
  Lines          6352     6596     +244     
  Branches       1255     1260       +5     
============================================
+ Hits           5012     5194     +182     
- Misses          867      927      +60     
- Partials        473      475       +2     

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