Add clustering options to `scattermapbox`

[archmoj]

Follow up for PR #5334.

Options added by @elben10 to enable mapbox clustering layer. Please note that currently it doesn't support the fill, selection and text features.

Some remaining TODOs:

• [ ] add jasmine tests to ensure Plotly.react can switch between modes i.e. with/without cluster
• [ ] add second mock & baseline using new attributes e.g. step and opacity

@plotly/plotly_js