add custom symbols for scattermapbox

[zach-morris]

This is a request to add functionality for custom markers / symbols in the scattermapbox library.

As it is, the documentation currently says that there are available custom markers available from Maki Icons.

In practice, these icons don't appear to work at all if any theming is present in your application. See here.

The request is to update functionality to allow use of custom marker symbols via svg, similar to Leaflets implementation. This will remove the issue of using the above icons since an SVG could directly be called.

[gvwilson]

@archmoj is this something that your recent work will address or is it still a new feature?

[archmoj]

One may be able to add these to scattermap not scattermapbox in future.
cc: @birkskyum What do you think of this?

[birkskyum]

I think there's a good chance that #1804 is resolved for the map- traces, because of the way we explicit load maki icons if they're missing instead of relying on the style to list them as a resource to be loaded like the mapbox- traces

This might to relieve some of the need for custom icons, since it's mentioned as the premise behind this ticket.

MapLibre has a way to use a custom icon for a marker ( https://maplibre.org/maplibre-gl-js/docs/examples/custom-marker-icons/ ), it just doesn't have a plotly api around it.