Adds hydra_zen.kwargs_of

[rsokl]

(Very excited about this one!)

This PR adds a new config-creation function:

>>> from inspect import signature
>>> from hydra_zen import kwargs_of, instantiate

>>> Config = kwargs_of(lambda x, y: None)
>>> signature(Config)
<Signature (x:Any, y: Any) -> None>
>>> config = Config(x=1, y=2)
>>> config
kwargs_of_lambda(x=1, y=2)
>>> instantiate(config)
{'x': 1, 'y': 2}

Excluding the first parameter from the target's signature:

>>> Config = kwargs_of(lambda *, x, y: None, zen_exclude=[0])
>>> signature(Config)
<Signature (y: Any) -> None>
>>> instantiate(Config(y=88))
{'y': 88}

Like builds and just, kwargs_of is a classmethod of BuildsFn and thus is customizable accordingly.

This function is powerful because it decouples a config from the target, meaning that the target need not be importable nor do you ever have to call the target.