mbgl::style::Style::getJSON should update according to runtime styling

[brunoabinader]

In GL JS, map.getStyle() invokes style.serialize(), which serializes the current state stylesheet state. This means that every runtime styling change is going to affect the layer property values on the returned JSON object.

In GL Native, mbgl::style::Style::getJSON returns the same original style JSON set via either mbgl::style::Style::loadJSON or mbgl::style::Style::getURL.

We should follow GJ JS approach and keep the returned JSON up-to-date with the runtime styling changes.

[brunoabinader]

/cc @mapbox/gl-core

[brunoabinader]

To accomplish this, we could adopt the following plan:

1. Add mbgl::style::Layer::asJSON() and mbgl::style::Source::asJSON(), and generate mbgl::style::Style::getJSON on the fly, or

2. Adopt source/layer update observers in the style object that would eventually update the internal JSON stylesheet upon changes

Option 2) is tricky because it requires traversing the JSON to update individual entries on each runtime style change, but provides O(1) results for getJSON. Thus, I find option 1) more compelling as it keeps parity with GL JS implementation, and getJSON does not seem like a function that needs to be called multiple times.

Adding {Layer,Source}::asJSON requires a serialization mechanism similar to Conversion::stringify.

[jfirebaugh]

Duplicate of #7563.