Create AudioStreamEffect

[RoyBerardo]

This adds a new AudioStream type, AudioStreamEffect, which applies audio bus effects directly to audio streams.

This partially addresses godotengine/godot-proposals#8190 ("Partially" because i believe it would be useful to apply effects to audio stream players, but that is a whole other topic)

Effects can be individually bypassed. By enabling process_bypassed_effects, bypassed effects will still process so bypassing and un-bypassing effects like reverb won't result in a burst of sound.

When the child stream is finished playing, this stream fades out for the duration of tail_time, allowing effects like reverb and delay to fade out.

This is currently a draft because I feel it would be useful to get community input on this new stream type. Some things I'd like to get some thoughts/discussion on in particular:

• Should process_bypassed_effects be per-effect? That would allow for more granular control but I worry about cluttering the effect list.
• Instead of tail_fade_curve, should there instead just be a bool that simply enables/disables a fade on the tail?
• Would appreciate it if someone looked over the docs I made, I'm not the best at writing those.
• AudioEffect docs would need to be updated since they talk about audio effects only in the context of busses.