Republicize DirectionsOptions.urlQueryItems

[1ec5]

Made DirectionsOptions.urlQueryItems open once again.

Fixes #460.

/cc @mapbox/navigation-ios @jmkiley @captainbarbosa @geografa

[1ec5]

Once this PR lands, you’ll be able to try out the Mapbox Directions API’s depart_at parameter (currently in beta) by subclassing RouteOptions (or NavigationRouteOptions if you’re using the navigation SDK). For example, this RouteOptions subclass requests directions for a trip that would begin an hour from now:

// Subclass NavigationRouteOptions instead if using the navigation SDK
class DeferredRouteOptions: RouteOptions {
    var startDate = Date()
    
    override open var urlQueryItems: [URLQueryItem] {
        let startTimestamp = ISO8601DateFormatter().string(from: startDate, timeZone: .autoupdatingCurrent)
        return super.urlQueryItems + [
            URLQueryItem(name: "depart_at", value: startTimestamp),
        ]
    }
}

let options = DeferredRouteOptions(coordinates: coordinates, profileIdentifier: .automobile)
options.startDate = Date() + 60 * 60
…