Better support of Specification Extensions for callbacks, paths and responses

[rafalkrupinski]

Currently callbacks, paths and responses are modelled as dicts, but that breaks support for specification extensions.

An alternative solution would be to use before-validation to catch all non-extension entries and put them in a dict inside the model, and leave extensions to be handled by extra. Basically all dynamic names would be pushed down into a dict. This would improve compatibility with python but break 1-1 mapping with the JSON representation, and would need to be coupled with pre-processing during serialization.

class Paths(BaseModel)
  paths = dict[str, PathItem]

  if PYDANTIC_V2:
    @validate_model
    def validate():...
    @model_serializer
    def serialize():...
  else:
    class Config:
      json_loads = load_paths
      json_dumps = dump_paths

I've done something roughly similar but only for V2 and only for parsing.