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.

The issue names no files or tests. Start by tracing how plotly.py serializes mapbox_layers and how the JavaScript map requests raster tiles, then compare that path with the linked transformRequest example. Done means custom headers can be supplied for each Mapbox layer tile request and the behavior is covered by appropriate tests.