Adds support for cross-draft validation

[fdutton]

Resolves #778

[codecov-commenter]

Codecov Report

Merging #779 (4bae15f) into master (1834353) will decrease coverage by 0.10%.
The diff coverage is 89.28%.

❗ Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the Github App Integration for your organization. Read more.

@@             Coverage Diff              @@
##             master     #779      +/-   ##
============================================
- Coverage     75.14%   75.05%   -0.10%     
+ Complexity     1191     1185       -6     
============================================
  Files           122      122              
  Lines          4108     4097      -11     
  Branches        792      787       -5     
============================================
- Hits           3087     3075      -12     
- Misses          696      698       +2     
+ Partials        325      324       -1     

Impacted Files	Coverage Δ
...n/java/com/networknt/schema/JsonSchemaFactory.java	78.77% <83.33%> (-0.20%)	⬇️
...rc/main/java/com/networknt/schema/SpecVersion.java	87.50% <92.30%> (-3.41%)	⬇️
...java/com/networknt/schema/SpecVersionDetector.java	100.00% <100.00%> (+4.16%)	⬆️

... and 1 file with indirect coverage changes