Splitting EXT_mesh_features into multiple extensions

[lilleyse]

This PR splits EXT_mesh_features into three separate extensions:

• EXT_mesh_features that only defines the concept of feature IDs
• EXT_structural_metadata that allows associating the features with metadata
• EXT_instance_features that supports GPU instance feature IDs

This was done for multiple reasons

• The extension was doing too much
• Feature identification and metadata can be cleanly decoupled
• Different metadata encodings may emerge in the future
• Several users of the extension are interested in feature IDs but want to use their own database

We even considered splitting EXT_structural_metadata into four separate extensions - one for the schema definition and three others for the encodings (property tables, property textures, property attributes) - but decided it was better to keep them together.

Opening the PR here for early feedback, then we'll updated the Khronos/glTF PRs. This includes the work from #51 #52 #55

To do:

• Dictionary vs. array form for featureIds
• From @ptrgags: When the property table offset/scale overrides the parent, should those be checked in lockstep? E.g. what if the class defined offset but not scale and the property table defines scale but not offset? even though the property table's offset is undefined, a default (probably 0) would make more sense than inheriting the class offset (which could be different)
• Allow propertyAttributes to refer to components of an accessor. Allows properties to be packed into the same attribute without excessive padding (glTF requires 4-byte padding for attribute elements)

[javagl]

The extensions that this PR consists of had originally been a single extension. The original, single extension has been reviewed. Each PR that carved out one of the new extensions has been created in a dedicated PR that went into the branch that this PR is based on. There have been changes in each individual part, but these should be consistent with the overall structure and description. So I think this could be merged.

Only one question: There are some links like https://github.com/CesiumGS/3d-tiles/blob/extension-revisions/next/REVISION_HISTORY.md that refer to a branch (and not main) - at which point are these supposed to be be updated?