[core] Add Layer::serialize() method

[alexshalamov]

New method allows serialization of a layer into a Value type, including all modifications done via runtime style API. New method is also an enabler for Style object serialization (sources, layers, etc). PR also fixes several issues in universal style setter / getter code.

Related issues: #7600
Fixes: https://github.com/mapbox/mapbox-gl-native-team/issues/194

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• document any changes to public APIs
• tagged @mapbox/maps-android @mapbox/maps-ios @mapbox/core-sdk if this PR adds or updates a public API
• apply needs changelog label if a changelog is needed (remove label when added)

/cc @mapbox/core-sdk

[pozdnyakov]

LGTM

[chloekraw]

This is great! cc/ @mapbox/maps-android @mapbox/maps-ios

New method is also an enabler for Style object serialization (sources

@alexshalamov does it already work if you want to serialize a runtime source or light object into a Value type, or are you saying these could be enabled in a follow-on pr?

[alexshalamov]

does it already work if you want to serialize a runtime source or light object

Not without a big effort from a developer. If and when we would enable full Style object serialization, developers would be able to:

1. map.loadStyleUrl(streets_base_style)
2. Do modifications via runtime styling, map.getStyle().addLayer() or map.getStyle().getLayer().setLayerProperty()
3. Serialize, and maybe store whole Style object. serializedStyle = map.getStyle().serialize();
4. On a subsequent start, load serialized style, map.loadStyle(serialized);

@chloekraw If source and light serialization would be required, yes, we could try adding that functionality in a follow-up releases / PRs.

[chloekraw]

@alexshalamov thanks! for now, could we open tickets to track the work? I think source would be useful, less sure about light.

[tobrun]

@chloekraw If source and light serialization would be required

Serialization support for sources would be great, light isn't used that much. To me, in the long run, it would be great to be fully compatible with style-spec. This will enable using json across style API with the mbgl value conversion system. This will simplify the generated API downstream even more and make implementing additional APIs on top more flexible.