[Enhancement] Add heatmap visualization style for Related Table-based context layers

[janbur]

Overview

This can be implemented after: #141

Also linked to: #145

Heatmaps are a great way to visualize point data, indicating where are the hot spots.

Solution

Add a heatmap visualization style for point context layers based on the value from an attribute. As GeoSight is using MapLibre, we could reuse: https://maplibre.org/maplibre-gl-js-docs/example/heatmap-layer/

The heatmap style should be defined at the Context Layer style config, either globally or overwritten at the project level, just like any other context layer style or label.