Add route selection by index

Nobody has claimed this yet.

**Mapbox Navigation SDK version:** 2.1.0

EDIT: grammar

Follow up work from https://github.com/mapbox/mapbox-navigation-android/pull/4892. Anticipating this issue will require refactoring and testing in order to get it all just right. It is considered out of scope of the initial integration of continuous alternatives because we may not want to do this. Re-ordering the list and assuming route index zero also works.

Here is the existing way to change the route leg in MapboxNavigation
``` kotlin
fun navigateNextRouteLeg(callback: LegIndexUpdatedCallback) {
arrivalProgressObserver.navigateNextRouteLeg(callback)
}
```

So here we're proposing adding and/or changing the functions in MapboxNavigation
``` kotlin
// Adding routeIndex may be semver, since it changes the position of the setRoute arguments
fun setRoutes(routes: List, routeIndex: Int = 0, initialLegIndex: Int = 0) {

// After routes have been set, be able to change the route index with something like this.
fun setPrimaryRoute(routeIndex: Int)
fun setPrimaryRoute(route: DirectionsRoute)
```

## Ui code to select a route

[Before](https://github.com/mapbox/mapbox-navigation-android/blob/ac985d5298f3b9dae22a8dae2eb74a5366ed7c1e/qa-test-app/src/main/java/com/mapbox/navigation/qa_test_app/view/AlternativeRouteActivity.kt#L273-L293)
``` kotlin
private val mapClickListener = OnMapClickListener { point ->
CoroutineScope(Dispatchers.Main).launch {
val result = routeLineApi.findClosestRoute(
point,
binding.mapView.getMapboxMap(),
routeClickPadding
)

val routeFound = result.value?.route
if (routeFound != null && routeFound != routeLineApi.getPrimaryRoute()) {
val reOrderedRoutes = routeLineApi.getRoutes()
.filter { it != routeFound }
.toMutableList()
.also {
it.add(0, routeFound)
}
mapboxNavigation.setRoutes(reOrderedRoutes)
}
}
false
}
```

After
``` kotlin
private val mapClickListener = OnMapClickListener { point ->
CoroutineScope(Dispatchers.Main).launch {
val result = routeLineApi.findClosestRoute(
point,
binding.mapView.getMapboxMap(),
routeClickPadding
)

result.value?.route?.let { mapboxNavigation.setPrimaryRoute(it)
}
false
}
```

Start with the MapboxNavigation entry point and the existing navigateNextRouteLeg and setRoutes APIs. Compare the current AlternativeRouteActivity implementation with the proposed setPrimaryRoute usage to understand route selection behavior. Done means supporting primary-route selection by index or DirectionsRoute while preserving route and leg handling, with refactoring and tests covering the new behavior.

Written by the indexing model from the issue text.