Make the signature of load_video_frames stricter

[AllenDowney]

Currently load_video_frames can take a VideoLoaderConfig object as a parameter or, if it's missing, it gathers the kwargs and makes a VideoLoaderConfig. This permissiveness complicates the implementation if npy_cache and wastes a few cycles validating the configuration over and over. It would be better to require a validated (once and for all) VideoLoaderConfig.

To make this change, we might have to add a deprecation warning now, fix some tests that rely on the current signature, and then enforce the stricter signature in the future.

[AllenDowney]

Stashing this snippet:

warnings.warn(
            "Calling load_video_frames with keyword arguments is deprecated. "
            "Pass a VideoLoaderConfig as the second parameter instead.",
            DeprecationWarning,
            stacklevel=2
        )