[style] Improve update layer support

[nishant-karajgikar]

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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.
• 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>Allow a developer to synchronously update a layer with one API call -- no longer have to retrieve and re-add a layer.</changelog>.

Summary of changes

This PR improves the way layers can be updated by introducing a new updateLayer() method which accepts a closure as a parameter. The method takes care of retrieving the layer from the style, passing it to the mutating closure, then applies the changes to the style.

User impact (optional)

Updating a layer is much simpler now:

self.mapView.style.updateLayer(id: "land", type: BackgroundLayer.self) { (layer) in
    layer.paint?.backgroundColor = .constant(.init(color: .blue))
}

[codecov]

Codecov Report

Merging #85 (02ccc29) into main (27d85bf) will decrease coverage by 28.22%.
The diff coverage is 6.25%.

@@             Coverage Diff             @@
##             main      #85       +/-   ##
===========================================
- Coverage   77.16%   48.93%   -28.23%     
===========================================
  Files         101      101               
  Lines        5658     5683       +25     
===========================================
- Hits         4366     2781     -1585     
- Misses       1292     2902     +1610     

Flag	Coverage Δ
Examples	?
unit-tests	48.93% <6.25%> (-0.31%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
Mapbox/MapboxMapsStyle/Style.swift	4.86% <0.00%> (-48.93%)	⬇️
Mapbox/MapboxMapsStyle/Types/Value.swift	61.11% <50.00%> (-22.23%)	⬇️
Mapbox/MapboxMapsStyle/Layers.swift	0.00% <0.00%> (-100.00%)	⬇️
Mapbox/MapboxMapsStyle/Generated/Terrain.swift	0.00% <0.00%> (-100.00%)	⬇️
Mapbox/MapboxMapsFoundation/AccountManager.swift	0.00% <0.00%> (-100.00%)	⬇️
Mapbox/MapboxMapsFoundation/MapboxLogoView.swift	1.02% <0.00%> (-97.58%)	⬇️
...rnaments/ScaleBar/MapboxScaleBarOrnamentView.swift	16.26% <0.00%> (-73.21%)	⬇️
.../Events/MapboxMobileEvents+TelemetryProtocol.swift	0.00% <0.00%> (-66.67%)	⬇️
Mapbox/MapboxMapsSnapshot/Snapshotter.swift	0.00% <0.00%> (-49.51%)	⬇️
...pboxMapsFoundation/Events/EventsStringTokens.swift	0.00% <0.00%> (-43.75%)	⬇️
... and 21 more