Expose AudioServer stream playback functions

[Spartan322]

• Closes Expose AudioServer methods which allow playing sounds to GDScript godot-proposals#14259

Since MixTarget needs to be handled by the exposed start_playback_stream now, I moved it to AudioServer, AudioStreamPlayer needs AudioServer anyway, so I mimicked the compatibility resolution in #116839.

Code Example:

extends Node2D

const test_stream : AudioStreamMP3 = preload("res://test.mp3")

func _on_button_pressed() -> void:
	var playback := test_stream.instantiate_playback()
	AudioServer.start_playback_stream(playback, &"Master")
	await get_tree().create_timer(1).timeout
	AudioServer.stop_playback_stream(playback)

Demo Project

expose-audio-server-playback-test.zip

Consideration

• Should this implement start_playback_stream functions whose signatures are:

void start_playback_stream(
	Ref<AudioStreamPlayback> p_playback,
	const Dictionary &p_bus_volumes, // Dictionary[StringName, Array[Vector2]]
	float p_start_time = 0,
	float p_pitch_scale = 1,
	float p_highshelf_gain = 0,
	float p_attenuation_cutoff_hz = 0
);

void start_playback_stream(
	Ref<AudioStreamPlayback> p_playback,
	const StringName &p_bus,
	const TypedArray<Vector2> &p_volume_db_vector,
	float p_start_time = 0,
	float p_pitch_scale = 1
);