Snapshotter

[evil159]

What does this pull request do?

This PR exposes Snapshotter and introduces refactored event handing for MapWidget that is also reused for Snapshotter. Event listeners(map loaded, style loaded, map idle etc.) are no longer being ignored after MapWidget state initialization, they can be updated dynamically.

Based https://github.com/mapbox/mapbox-maps-flutter/pull/386. Addresses https://github.com/mapbox/mapbox-maps-flutter/issues/210, https://github.com/mapbox/mapbox-maps-flutter/issues/279, https://mapbox.atlassian.net/browse/MAPSFLT-159

What is the motivation and context behind this change?

Snapshotter provides a way to get an image snapshot of a map without instantiating a MapWidget

Pull request checklist:

• [x] Add a changelog entry.
• [x] Write tests for all new functionality. If tests were not written, please explain why.
• [x] Add documentation comments for any added or updated public APIs.

[pjleonard37]

Looks great, @evil159! 📸