[modosc]

part of #210.

• parses components.mediaTypes
• in 3.2 docs, request body / response content resolves $refs, so bodies validate against the referenced media type
• a $ref that doesn't resolve raises under strict_reference_validation, otherwise it accepts any body
• 3.0/3.1 docs are unchanged: a $ref in content is still ignored
• adds allow_3_2_features: true to opt earlier docs into 3.2 runtime behavior (the discriminator defaultMapping and query / additionalOperations prs use it too)
• a $ref'd file with no openapi: field follows the doc that loaded it
• parameter/header content isn't modeled

[geemus]

lgtm