Add overview mode for MapboxViewportDataSource

Nobody has claimed this yet.

`MapboxViewportDataSource`'s overview state only frames the current leg of the route, which means that in case of multi-leg route, a portion of the route (everything besides the current leg) will not be framed.

There's a request to provide an option to frame the full route at all times.

My initial idea is to introduce an enum, sealed class or a similar interface that allows to select the overview mode:
```
sealed class OverviewMode {
object CurrentLeg: OverviewMode
object Full: OverviewMode
}
```

and accept it in `MapboxViewportDataSourceOptions#overviewFrameOptions`. Depending on the mode, we'll either frame the full route or the current leg.

Start at MapboxViewportDataSource and MapboxViewportDataSourceOptions#overviewFrameOptions; inspect how the overview state selects the current leg for framing. Confirm the API shape and route geometry used for full-route framing, then add coverage for both modes and verify that Full frames the complete multi-leg route.

Written by the indexing model from the issue text.