Support JSON any type

[rafalkrupinski]

When the return type is unknown at the coding time, allow returning values as a type hint representing any JSON value.

Any JSON value is one of

• None
• str
• int
• float
• list of any JSON values
• model class with all fields of any JSON value type (object).

This type could be used for any header or model field, including nested fields. Using it denotes an unknown type (no type in OpenAPI schema) and must not trigger UnexpectedResponse.