Add expression accessors

[tobrun]

Tailwork of #10654, allow to get an existing layout/paint porperty or filter expression from a layer.

Current value/function api:

assertNotNull(layer.getSymbolPlacement());
assertNotNull(layer.getSymbolPlacement().getFunction());
assertEquals(CameraFunction.class, layer.getSymbolPlacement().getFunction().getClass());
assertEquals(IntervalStops.class, layer.getSymbolPlacement().getFunction().getStops().getClass());
assertEquals(1, ((IntervalStops) layer.getSymbolPlacement().getFunction().getStops()).size());

[1ec5]

#10726 implemented some accessors in mbgl that could be useful for implementing fully typed accessors in Java. #10714 could also help, but that approach would probably not benefit from compile-time type checking.

[1ec5]

#10726 implemented some accessors in mbgl that could be useful for implementing fully typed accessors in Java.

Here’s how the iOS/macOS SDK is using these accessors:

mapbox-gl-native/platform/darwin/src/MGLStyleValue.mm

Line 44 in 082df68

id MGLJSONObjectFromMBGLExpression(const mbgl::style::expression::Expression &mbglExpression) {

[tobrun]

Landed in #11352