Style JSON configuration on map snapshotter

[tobrun]

Closes #11974 , this PR adds the ability to use a local style.json file as style for a map snapshotter. This option is needed to port our node render tests to Android in #10047.

@LukasPaczos would you be able to review the java code?
@ChrisLoer would you be able to review the core code? (I'm not satisfied with current optional setup, is there a way to make this cleaner? variant or other c++ construct?)

[ChrisLoer]

Hmm yeah, that use of the optional is awkward, although I can see why you did it. I think ideally the arguments wouldn't be string typed so the code could know what to do (something like "variant<URL, JSON>"), but maybe a cheap way to approximate that is just to make a single style argument that's a std::pair with a bool indicating whether it's JSON or a URL? It'd still be a bit awkward, but it would avoid the unused parameters (e.g. having to provide a "styleURL" that will be ignored).

[tobrun]

@ChrisLoer in 73d6dcf, I went with the std::pair<bool, std::string> approach. Lmk what you think, if 👍 I will update the other platforms.

[ChrisLoer]

@tobrun yeah, I think that's an improvement. Everything else on the core side looks good. 👍