(Payload section -> APIE) Generalized mediaType handling

[tjanc]

During the translation of an API Blueprint Payload Section to corresponding Asset Elements,

• content types are fully interpreted (parsed, evaluated, serialized)
  • uses PEGTL grammar
  • implements media type serialization (unit tests needed)
• translation of user-supplied Body Section & Schema Section into Asset Elements is performed in isolation from generating and attaching derived Asset Elements.
  • prerequisite to implement drafter options generate-json-values & generate-json-schemas

Impact on tests

• Behaviour now corresponds to generate-json-values=true, generate-json-schemas=true
  • test/fixtures/api/action-request-attributes.json
  • adds generated JSON Schema
  • test/fixtures/api/issue-702.json
  • adds generated JSON
  • adds generated JSON Schema
  • test/fixtures/api/schema-body.json
  • fixes class of a JSON schema asset to equal messageBodySchema where previously it was messageBody
  • formats media type of that schema (removes white space not required)
  • test/fixtures/api/schema-custom-json.json
  • adds generated JSON Schema
  • test/fixtures/api/schema-custom.json
  • fixes contentType of a custom schema asset to equal application/schema+xml where previously it was application/xml
  • test/fixtures/extend/many.json
  • fixes propagation of media type parameters (charset=utf-8) where they were previously omitted
  • test/fixtures/render/action-request-attributes.json
  • adds generated JSON Schema
  • test/fixtures/render/content-type.json
  • fixes propagation of media type parameters (charset=utf-8) where they were previously omitted

[tjanc]

Closing in favor of #780 .