Make lapidary work without generated model

[rafalkrupinski]

Having generated model classes and method stubs is great but it should be optional.

Stubs are optional by design, they're only used by development tools. [no more stubs]

Once Lapidary validate models directly against the schema (#38) it should be possible to accept any model, e.q. a class instance, named tuple or plan dict. [not planning validation]

in case of class instances and perhaps named tuples, serialization to JSON might require consulting the openapi document to check for custom names (using x-lapidary-name).
Skipping support for dict would simplify the code, as it would require support for just a single interface (hasattr/getattr).

Response bodies could be returned as a simple named tuples, as they are fast, read-only and we don't use inheritance.

TODO: check if it's OK with mypy to have one type (class) in .pyi and another (NamedTuple) returned at run-time.

Using model types is convenience, but lapidary should support sending and receiving simple dictsin case of json content type.