Support Ducking on AudioStreamRandomizer

[avanwinkle]

GMC currently does not support ducking audio that is being played from an AudioStreamRandomizer node because ducking requires knowledge of the duration of the audio stream and AudioStreamRandomizer does not expose the randomly-selected stream.

There is a PR open (godotengine/godot#88437) to expose the stream, and if/when that merges and is released, the ducking code will be able to retrieve the stream and duck it appropriately.

This ticket is to track the progress of that PR and update the ducking after it merges.