Add identifier to MapMatchingResponse

Nobody has claimed this yet.

Per mapbox/mapbox-java#1174, the Mapbox Map Matching API will start returning a `uuid` field for all successful map matching requests. MapMatchingResponse should have an `identifier` property similar to the existing `RouteResponse.identifier` property, decoding it when `uuid` is present in the response.

https://github.com/mapbox/mapbox-directions-swift/blob/b7c26f516cdc46637e5a2eb5b775694516f8b7b3/Sources/MapboxDirections/RouteResponse.swift#L11 https://github.com/mapbox/mapbox-directions-swift/blob/b7c26f516cdc46637e5a2eb5b775694516f8b7b3/Sources/MapboxDirections/RouteResponse.swift#L33

`RouteResponse(matching:options:credentials:)` should pass in the value of this new property instead of `nil`:

https://github.com/mapbox/mapbox-directions-swift/blob/b7c26f516cdc46637e5a2eb5b775694516f8b7b3/Sources/MapboxDirections/RouteResponse.swift#L71

Unlike mapbox/mapbox-java#1174, this work is not blocked by deploying the change to all profiles, because the `identifier` property would be optional.

/cc @mapbox/navigation-ios @danpaz

Start with Sources/MapboxDirections/RouteResponse.swift and its existing identifier property and initializer. Trace how MapMatchingResponse is decoded and how RouteResponse(matching:options:credentials:) is constructed. Done means an optional identifier is decoded from the response's uuid field and passed into the matching RouteResponse.

Written by the indexing model from the issue text.