KHR_audio_emitter: Add pitchSpeed and make audio source's data optional

[aaronfranke]

This PR has 2 changes in it:

• Add a new "pitchSpeed" property to audio sources.
  • I swear we discussed this before in Discord meetings, but I guess we forgot about it?
  • It was suggested here: KHR_audio_emitter KhronosGroup/glTF#2137 (comment)
  • I would personally find this field useful, and it's supported in Godot and WebAudio.
  • Having this property aligns with WebAudio in all but name, which calls it playbackRate: https://developer.mozilla.org/en-US/docs/Web/API/AudioBufferSourceNode/playbackRate
  • I don't really care what name we use. I do think having it contain the word "pitch" is valuable because users who aren't familiar with audio may expect it to resample and don't understand why changing the playback rate would "cause the pitch to change", but even then I'm good with whatever consensus we bikeshed over this.
• Make an audio source's audio data field optional instead of required. Two reasons:
  • This technically breaks extensions such as "OMI_audio_ogg_vorbis" because they provide their own audio data separately in a new field. But if the built-in field is empty, it's technically invalid with the spec in master.
  • Making audio data mandatory means it's not possible to define gain or pitchSpeed or autoPlay on an emitter, because it's not allowed to make a placeholder audio source with no data.
  • Making this property optional fixes both of these problems.

[yankscally]

pitchSpeed... in one way, the name is illogical, but in another way, I can eyeball it and know what it means - unlike playbackRate. In the audio realm, this has many names also, so there is no concrete naming scheme for this, it's usually whatever makes the most sense in that context.

Approved.