[Feature request]: expose alternative-route data + programmatic route selection (Android Auto / CarPlay can't select alternatives)

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Use case

When `RoutingOptions.alternateRoutesStrategy` is `all`, the native `NavigationView` draws the alternative routes and lets the user tap one to switch the active route (firing setOnRouteChangedListener). However, the SDK exposes no way for an app to (a) read the computed alternatives, or (b) select one programmatically. This makes alternative-route selection impossible on any surface where the built-in `NavigationView` tap isn't available — most importantly, Android Auto (and CarPlay), whose map surface is render-only.

### Proposal

Implement an API to select an alternative route on the auto screen.