Add volume_db to AudioStream

[RoyBerardo]

This adds a volume_db property to all AudioStreams. For wav, mp3, and ogg AudioStreams, this property is set during import.

All AudioStreams can have their volume_db property set during runtime.

Changing volume_db while audio plays should not result in clicks because the value is interpolated over the course of one buffer of audio frames.

This closes godotengine/godot-proposals#12133

[RoyBerardo]

Going to try to squeeze as much performance out of this as I can. Converting to draft for now.

[RoyBerardo]

Made some pretty big optimizations to this, thanks to some good suggestions from a friend. I did a test with a lot of AudioStreams all playing at once. It had 128 AudioStreamPlayer3Ds, all with AudioStreamSynchronized, each with lots of other simultaneously-playing streams. Each stream had its volume_db set to 0, so no volume adjustment was actually applied. Here are the (approximate) measurements of the audio thread from the project's profiler:

Godot without this PR: 2.1ms
This original PR: 2.79ms
This PR with these optimizations : 2.15ms

With optimizations, that's approximately a 2.4% increase from where Godot currently is, as opposed to a 32.9% increase with this PR's original code. I should mention that I don't feel these tests were the most accurate, but they definitely get the numbers in the ballpark.