[18.0][ADD] fleet_maps: new module

[douglascstd]

New module fleet_maps: a generic map view for fleet vehicles, based on
web_view_leaflet_map.

Features

• A leaflet_map view for fleet.vehicle, using stored map fields:
  latitude, longitude, marker title, marker address and last update.
• A Fleet Map action and menu entry under Fleet > Vehicles, showing
  only the vehicles that have a position.
• A smart button and a Map page in the vehicle form, with an embedded
  OpenStreetMap preview.
• Extension point _get_map_position_values() so other modules (e.g.
  telemetry integrations) can provide the position data without depending
  on a specific map implementation.

Tests

Unit tests cover the default computed values, the map preview with and
without position, the actions and the data records.

How to test

1. Install fleet_maps.
2. Go to Fleet > Vehicles > Fleet Map: only vehicles with a position are
   shown.
3. Open a vehicle form: a Map smart button and a Map page with the
   OpenStreetMap preview are available.