Add support for feature state APIs

[jmalanen]

Fixes: #11846

[asheemmamoowala]

Just have a couple notes on the implementation choices:

• The use of std::map and std::unrodered_map is likely to provide unsatisfactory performance when there are large changes in feature state - for example: https://bl.ocks.org/asheemmamoowala/014fa054005f3a46a10fd6fc306f551f

• The use of Convertible as the input parameter for setFeatureState makes it very broad, and hard to force type checking on the input data. The platform APIs will probably want to expose this functionality using platform-appropriate types (for example NSMutableDictionary on osx) which would create an extra step to make those into Convertible objects.

cc @julianrex @tobrun for input on the interface for platform SDKs.

[jmalanen]

Adding a screenshot of feature state demo on mbgl-glfw app. The demo implements the same hover effect as this GL-JS example.

[alexshalamov]

Personally, I prefer old style (chromium) where each argument is on a new line. @tmpsantos could we add exception for this to clang-format config?

[alexshalamov]

@tmpsantos could we add these to clang format?
BinPackParameters: false
AllowAllArgumentsOnNextLine: false

[pozdnyakov]

We've agreed with @jmalanen that he will investigate moving feature state into GeometryTileFeature