Add style json in MLNMapView

[wangyingfang]

Address issues 3052 - Support initializing map with string JSON style on iOS

[github-actions]

Bloaty Results (iOS) 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0% +1.38Ki  [ = ]       0    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results-ios/pr-3240-compared-to-main.txt

[random3940]

Assuming that it's the same API as we have on Android - LGTM.

[wangyingfang]

@louwers please help review and re-run the pipelines.

[louwers]

If approach will first load the default style. It is possible that the user will first see the default style flash before this style is finally loaded.

[louwers]

We're already parsing the style when loading the style, so we parse the JSON another time.

This does not check if the style is really valid, it only checks if it is valid JSON.