Implement tooling for RouteOverlay customization on iOS

[Archdoog]

#439

[ianthetechie]

Thanks! I'll take a look at this later today and either whip up a quick PR or document what needs to be done if it's not a super quick one.

[ianthetechie]

As @ben-burwood already pointed out, we already have something close to this with the RouteStyleLayer. Currently we render both lines in the NavigationMapView body.

I suspect what we probably want to at some point is:

1. Make RouteStyleLayer handle drawing both the traveled and remaining route line. It feels like this should contain the full default implementation in a single style layer collection (the SwiftUI DSL is more advanced here).
2. Accept another MapContentBuilder parameter with a default value that creates the built-in RouteStyleLayer.
3. Obviously, we should also make NavigationState's remainingRoutePolyline return what it's supposed to ;)