Webapp needed info

Since the frontend was a fork of WHIS there is already a defined structure for the API responses.
The app relies on a standard STAC api with the following details:

One collection with the geometries of the AOIs, where each item is an AOI and has a `name` and an `id`. For WHIS we have the volume of each lake, but here we can replace with something else, or remove it.

A second collection with all the data for the AOIs, where each item corresponds to an AOI and a date. (item named `_`). Each item has one asset for each indicator we want to show, where for each asset we have (`cog`, `mean`, `min`, `max`).

We also need a way to get information about the exiting indicators:
- id
- name
- values extent
- dates extent

In the case of WHIS this is somewhat hardcoded.

This is the structure that the app uses at the moment. There can be changes to better suit a more abstracted use case, but shouldn't be drastically different.

@emmanuelmathot @ciaransweet @pantierra Maybe let's discuss whether this is workable.