Refactor internal-only JSON:API extensions support

[bkoelman]

Refactors non-public methods for JSON:API extension support, for consumption by OpenAPI. Pass the containing ResourceType for attributes/relationships validation.

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• Adapted tests
• N/A: Documentation updated

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.69%. Comparing base (169ffee) to head (c777ba1).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1687   +/-   ##
=======================================
  Coverage   90.69%   90.69%           
=======================================
  Files         354      354           
  Lines       11536    11537    +1     
  Branches     1900     1901    +1     
=======================================
+ Hits        10462    10463    +1     
  Misses        702      702           
  Partials      372      372           

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