Updates to EXT_feature_metadata

[donmccurdy]

From #9 –

• Refine binary data storage to avoid breaking changes to upstream glTF 2.0
• Remove prefix: _FEATURE_ID_# → FEATURE_ID_#
• Define ID and NAME semantics directly in the document

[ptrgags]

Overall these changes look good, I had a couple points for discussion

[donmccurdy]

Converted this to a draft for the time being – @lilleyse if I understand correctly you'll start a branch off 3d-tiles-next and I'll rebase the PR onto that instead?

[donmccurdy]

Thanks! PR points to 3d-tiles-next-rev now, I think this might be ready to merge and I'll start a new PR for the next batch of changes?

/cc @ptrgags

[ptrgags]

@donmccurdy looks good, I'm going to merge this. Thanks!