allow custom HTTP headers (e.g. bearer token) to Mapbox layer source

Hi!

I wanted to display a Mapbox layer that requires authentication through bearer tokens, i.e. when doing something like:

fig.update_layout(
    mapbox_style="white-bg",
    mapbox_layers=[
        {
            "below": 'traces',
            "sourcetype": "raster",
            "sourceattribution": "My private ortho",
            "source": [
                "https://my_private_source.com/MapServer/tile/{z}/{y}/{x}"
            ]
        }
      ]
)

I need to pass custom HTTP headers to each tile request. In JavaScript, this can be easily achieved with the Mapbox API thanks to transformRequest (see here):

const map = new mapboxgl.Map({
  container: 'map',
  center: [2.35, 48.86],
  zoom: 13,
  transformRequest: (url, resourceType) => {
    if (url.startsWith('https://my_private_source.com')) {
      return {
        url: url,
        headers: { 'Authorization': 'Bearer ' + my_private_token }
      }
    }
  }
});

However, this does not seem supported by plotly.py, or am I missing something? With some pointers I could work on a PR, but I'm not sure where to start in the code base.

El issue no menciona archivos ni tests. Empieza rastreando cómo plotly.py serializa mapbox_layers y cómo el mapa de JavaScript solicita tiles ráster; después, compara ese flujo con el ejemplo enlazado de transformRequest. Se considera terminado cuando se puedan proporcionar headers personalizados para cada solicitud de tiles de una capa de Mapbox y el comportamiento esté cubierto por tests adecuados.