[Draft] Locator Overlays aka 'Heatmaps'.

[Bonkles]

This PR is a work-in-progress to demonstrate the following Proof-of-concept: locator overlays for Rapid Datasets.

The procedure is as follows:

1. Create a public vector data endpoint for the heatmap (an internal Meta process that I won't go into here).
2. Hook our Rapid System code to the overlay using the code in this PR.
3. Fire a new bit of pixi rendering code to draw very simple, highly transparent circle shapes for each point we get back from the vector heatmap endpoint.

This POC shows the resultant experience of zooming in at very high zooms over the continental US.

The heat map shown is for a pedestrian-related dataset that is centered over large cities:

https://github.com/facebook/Rapid/assets/1887955/8866be6d-437b-4b3c-a689-d280c6a9ffe7