A couple of enhancements

[rafalkrupinski]

Some enhancements I'd be happy port from my project:

• parametrized Reference (#39)
• ExtendableModel (#40, also validate extended properties start with 'x-' in 3.0)
• parameter style enum (currently it's str)
• validating that no more than one of example and examples is defined
• ModelWithAdditionalProperties (I use it for operations in PathItem, instead of separate fields)
• ModelWithPatternProperties (paths must start with '/', response keys constrained with regex)
• reference resolver (local only)
• exposing ParameterBase (parent of Parameter and Header)

~~Also, Paths could be changed to dict[str, PathItem | Reference], and ref could be removed from PathItem.~~ this is valid

• validate parameter has no more than one of content and schema