[MAPS3D-1645] StyleSpec: model source definition (#6204)

This PR extends **style specification** with full definition of **model
source** as well as **model-elevation-reference**.
Groundwork for removal of _non-experimental_ status of model layer
targeted at end of year x.17 gl-js/maps releases (MAPS3D-1645,
MAPS3D-1958).

The actual functionality on
- gl-native side is **already there**
- gl-js side missing features are implemented as part of
#5567.

**Changes in this PR**
- definition of `model source`
- documentation / preparation for style-spec docs in [docs-root
PR](mapbox/subdomain-docs-root#486)
- moved `model-elevation-reference` from gl-native-only to v8.json

> [!NOTE]
> Experimental/private flags are kept until functionality is available
in main.

**Public GL-JS commit message**
<public>Extended style specification with model source
definition.</public>

cc @mapbox/gl-js
cc @mapbox/gl-native

cc @mapbox/maps-ios
cc @mapbox/maps-android

---------

Co-authored-by: Ivan Persidskii <[email protected]>
GitOrigin-RevId: 3b7ba4d838b739216a72a833e979dbf5278cc36f

Author: 2 people