Refactorings for JSON:API extensions to unblock OpenAPI support

[bkoelman]

Exposes internal-only extensibility points for validating and writing JSON:API extension data during serialization. This is needed for OpenAPI support, which requires our own JSON:API extension to handle resource inheritance.

Renamed JsonApiExtension to JsonApiMediaTypeExtension, to reserve the name for future use.

QUALITY CHECKLIST

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

[codecov]

Codecov Report

Attention: Patch coverage is 90.66667% with 7 lines in your changes missing coverage. Please review.

Project coverage is 90.73%. Comparing base (43c1a4b) to head (3840fcb).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
...lization/JsonConverters/ResourceObjectConverter.cs	92.50%	1 Missing and 2 partials ⚠️
.../JsonApiDotNetCore/Middleware/JsonApiMiddleware.cs	33.33%	0 Missing and 2 partials ⚠️
...c/JsonApiDotNetCore/Middleware/JsonApiMediaType.cs	88.88%	1 Missing ⚠️
...DotNetCore/Middleware/JsonApiMediaTypeExtension.cs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1629      +/-   ##
==========================================
+ Coverage   90.72%   90.73%   +0.01%     
==========================================
  Files         352      353       +1     
  Lines       11447    11492      +45     
  Branches     1881     1888       +7     
==========================================
+ Hits        10385    10427      +42     
- Misses        695      696       +1     
- Partials      367      369       +2     

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