Publish vehicle speed through ferrostar core

[Archdoog]

To leverage any future location provider enhancements, we should publish a uniffied device speed through the location provider and state. This will allow a simple default implementation in the views as noted with #247

Created this so #89 can be closed with the current annotations MR #261

[ianthetechie]

What needs to be added here exactly? This is already part of the UserLocation struct which is published from the LocationProvider implementers (and contains GPS speed, not the less accurate derived speed; I should document that more clearly perhaps?).

It also ends up in TripState indirectly through SnappedLocation, but IMO this is better to get either directly from the location provider or, if needed in the UI, it of course ends up in UI state. On iOS, this would be accessible via snapped location, and on Android we now bubble up the "raw" location (which is more obvious than the snapped one) in the UI state as of #250.

TL;DR, I think this is already published, we can make views today based on the Speed model, and any UserLocation should already be publishing the more accurate speed info derived from GPS.