util.ImmutableValidatedObject: Support Optional[ImmutableValidatedObject]

[rehno-lindeque]

Similar to 88494fb this adds support for Optional[ImmutableValidatedObject]

Usage

• Support for optional sub-modules with required sub-options.
• Simplify auto-generating plugin code from API definitions that follow this pattern.

E.g. this is not currently possible:

{
  misc = mkOption {
    default = null;
    type = types.nullOr (types.submodule miscOptions);
  };
}

Considerations

Supporting additional generic types

It's trivial to add support forMapping[Any, ImmutableValidatedObject] as well, but I didn't want to add new code that isn't actively used in a plugin.

Satisfying mypy

Mypy needs to be upgraded to remove type: ignore[attr-defined] after typing.get_args and typing.get_origin.

Never-the-less the tests pass just fine with this version of python:

$ python --version
Python 3.8.7

[rehno-lindeque]

I discovered that the logic for this is slightly off, so hold off reviewing for now please.