Use speed limit from NavNative in turn-by-turn navigation

[azarovalex]

Right now in the active navigation we use the speed limit from the Directions response: https://github.com/mapbox/mapbox-navigation-ios/blob/89708651afae2e98ec774d7d2e00c7773aa87fcc/Sources/MapboxCoreNavigation/RouteLegProgress.swift#L209-L236

It's better to migrate to the values returned by NavNative, because they may contain speed limits from the route graph even if the Directions response is missing these values.

[1ec5]

We can introduce a new private property to RouteStepProgress to indicate the current speed limit. RouteController would set it and this RouteLegProgress property would return its value. The existing implementation can be a fallback in case the property hasn’t been set. For example, this would allow an application to get the speed limit at an arbitrary location along the route by constructing a RouteLegProgress, without having to navigate there first. (That said, this implementation isn’t especially complex, so the application could just inline the logic and rely on MapboxDirections functionality for it.)