Add layerId to AnnotationManager

[julianrex]

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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.
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• Add an entry inside this element for inclusion in the mapbox-maps-ios changelog: <changelog>Added function to get layer identifier for an annotation type.</changelog>.
• Update the migration guide, API Docs, Markdown files - Readme, Developing, etc

Summary of changes

This PR adds a function to get the layer id for a particular annotation type. It also adds a WIP convenience function (_moveLayer) to Style to allow layers to be moved. These combined give developers a method to move the underlying annotation layer.

The _moveLayer change is marked as :nodoc: and should be consider prone to change.

cc @neel

[julianrex]

cc @Chaoba @MaximAlien @jill-cardamon