Add get_duration method to Comfy VIDEO type

[christian-byrne]

Add get_duration to Comfy VIDEO type.

Can be used like:

class ExampleNode:
    @classmethod
    def INPUT_TYPES(s):
        return {
            "required": { "video": (IO.VIDEO, {}) },
        }

    FUNCTION = f
    def f(self, video):
        duration = video.get_duration()

Can be useful to validate video inputs before sending to API nodes without having to decode (most APIs have duration constraints for video inputs).