Add OMI_audio_emitter_extension

[robertlong]

Work in progress validation for the OMI_audio_emitter extension

[CLAassistant]

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0 out of 2 committers have signed the CLA.

:x: robertlong
:x: antpb
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[lexaknyazev]

@robertlong Thanks for the heads up! Are you planning to submit a PR with the OMI extensions to the main glTF repo?