frontend: map: allow setting a custom vehicle image

### Current behaviour

We [show default icons](https://github.com/bluerobotics/cockpit/pull/2226) on the map for the most common vehicle types Cockpit is used with, but not every vehicle from a given type looks the same, and some users / organisations may wish to use an image that specifically matches their vehicle.

### Expected or desired behaviour

Allow a user to provide a custom vehicle image (per vehicle type), which gets used as the icon on the map.

Ideally these would also be supported as [images](https://github.com/bluerobotics/cockpit/issues/2224) for [dynamic points of interest](https://github.com/bluerobotics/cockpit/pull/1945), so that they could be used to visualise [other vehicles](https://github.com/bluerobotics/cockpit/pull/1815) (e.g. in the same MAVLink network).

### Prerequisites

- [x] I have checked to make sure that a similar request has not already been filed or fixed.

Start by tracing the map vehicle icon implementation introduced by pull request 2226. Define how users provide an image per vehicle type, then verify that the selected image replaces the default map icon. Also assess whether the same images can support dynamic points of interest and other vehicles as described in issues 2224 and 1815.

Written by the indexing model from the issue text.