Enable users to draw custom objects

[javiber]

This PR addresses a specific use-case when the users want to draw a TrackedObject but they only have the data of the object and not the instance. This can happen when the TrackedObject is serialized and restored.

It's hard (if not impossible) for users to instantiate a TrackedObject directly when they want to draw it. With this change, the Drawable class can be instantiated directly and then drawn like so:

draw_boxes(
    frame,
    drawables=[
        Drawable(
            points=np.array([[20, 20], [50, 90]]),
            label="person",
            id=123,
            scores=np.array([0.88]),
        )
    ],
)