GLTF Export - Metadata support

[dpasukhi]

Implementing metadata support for GLTF format extends the functionality.
New metadata needs to be presented as a array of "key-value" attributes.
For the GLTF format there are a few ways to store metadata.
The easiest way to store them in the "nodes" section with "extras" section.
Some details from the forum by gkv311 n: https://dev.opencascade.org/node/87550#comment-25554

For the moment, RWGltf_CafWriter will write "extras" within "asset" section of the file (e.g. as metadata for entire file) when 'file info' string-string map is passed to Perform() method.

For writing some metadata within node elements (if this is what you are talking about), the glTF writer has to be improved. Note that XCAF document stores such metadata in form of 'named data' attributes, so that glTF writer would better fetch information from there.

The way how to store metadata within glTF file is unclear, though. "extras" sections looks like the most simplest approach, but with unclear compatibility across readers and writers. There are also cumbersome extensions like KHR_xmp and its successor KHR_xmp_json_ld, which are rather more complex to implement.

File samples can be found by the next search request: https://github.com/search?q=path%3A*.gltf+extras&type=code&ref=advsearch

Some discussion:
KhronosGroup/glTF#1387

[dpasukhi]

Integrated by #87