Expose setFeatureState / getFeatureState / removeFeatureState and add associated example

[nishant-karajgikar]

PRs must be submitted under the terms of our Contributor License Agreement CLA

Pull request checklist:

• Briefly describe the changes in this PR.
• Include before/after visuals or gifs if this PR includes visual changes.
• Write tests for all new functionality. If tests were not written, please explain why.
• Add example if relevant.
• 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></changelog>.
• Update the migration guide, API Docs, Markdown files - Readme, Developing, etc

Summary of changes

This PR adds setFeatureState, getFeatureState and removeFeatureState to the mapboxMap API surface. Each of these APIs are used to manage a stateMap that will be associated to a given feature.

Example:

Simulator.Screen.Recording.-.iPod.touch.7th.generation.-.2021-08-24.at.08.51.34.mp4

User impact (optional)

Developers can now use these APIs to add stateful information to each feature which can then be used to drive various use cases (like styling logic in expressions).