Move MapEvents on handling from BaseMapView to MapboxMap

[julianrex]

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Pull request checklist:

• Briefly describe the changes in this PR.
• Write tests for all new functionality. If tests were not written, please explain why.
• Add example if relevant. Updated
• Document any changes to public APIs.
• Apply changelog label ('breaking change', 'bug 🪲', 'build', 'docs', 'feature 🍏', 'performance ⚡', 'testing 💯') or use the label 'skip changelog'
• Add an entry inside this element for inclusion in the mapbox-maps-ios changelog: <changelog>Moved on()fromBaseMapViewtoMapboxMap.</changelog>.
• Update the migration guide, API Docs, Markdown files - Readme, Developing, etc

Summary of changes

This PR changes the behavior of on(_:handler:) to handle map events and moves the function to MapboxMap.

on(...) has been replaced by

• onNext(...) -> Cancelable
• onEvery(...) -> Cancelable

Assuming there is no retain cycle that stops the MapView or Snapshotter from being deallocated, then unsubscribing will be handled on your behalf when the MapView is deallocated: The underlying listener will unsubscribe from the map, and the closure will be released.

If you do have a retain cycle or you need to stop listening, for example if you want to listen to 3 events then stop, then call cancel on the returned Cancelable object.

Here's an example of how the call-site changes:

mapView.on(.styleLoaded) { event in ... }

becomes

mapView.mapboxMap.onNext(.styleLoaded) { (event) in ... }

cc @mapbox/maps-android

[nishant-karajgikar]

Do we need a test for this?

[julianrex]

There is..sort of. See below.

[julianrex]

FYI @nishant-karajgikar