Add kin-openapi parity features: router, content validation, and OpenAPI 3.2 support
Port key kin-openapi capabilities into libopenapi-validator:
- New router package with route resolution and server matching
- New content package for content-type negotiation and body decoding
- Content-based parameter validation (parameters/content_parameter.go)
- Internal bodycodec and requeststate packages for body/route caching
- Request defaults application (request_defaults.go)
- Validated OpenAPI 3.2 document validation support with new test specs
- Kin parity test suites across paths, requests, responses, and root
- Extended config options and updates across parameters, request,
- response, and schema validation paths.