Support components.mediaTypes for OpenAPI 3.2 (parse + MediaTypesBefore32 rule)

[modosc]

• 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 (also used by Support Discriminator defaultMapping for OpenAPI 3.2 (parse + runtime fallback + DefaultMappingBefore32 rule) #10 and Support query method and additionalOperations for OpenAPI 3.2 (parse + operation lookup + QueryMethodBefore32/AdditionalOperationsBefore32 rules) #11)
• a $ref'd file with no openapi: field follows the doc that loaded it
• parameter/header content isn't modeled

depends on #7.