add support for static image style parameters

[riastrad]

The Static Image API now allows clients to pass certain style parameters with static image requests. More context can be found on the public documentation pages: https://docs.mapbox.com/api/maps/#style-parameters.

Although this feature is currently in beta, it might be worthwhile to get a head start on adding support for these parameters to this library.

[1ec5]

The new style parameters require passing in a JSON blob, which is quite awkward to construct and manage in Swift. I think this library should provide a convenience function that generates the JSON blob via strongly typed properties. Perhaps we could adapt this map SDK script to generate the necessary API in this library.