Implement STAC catalog for Trends Earth outputs

[Samweli]

Part of #825

Implement a STAC catalog structure to organize Trends.Earth JSON outputs. The catalog should include a root catalog, collections for different datasets (e.g., NDVI, land degradation), and individual items representing dataset instances.

The catalog should support STAC collections, JSON to STAC items conversion, valid STAC metadata and also be compliant to the STAC specification standards.

Use pystac as a main tool for the catalog development.

This work will provide a ground for sustainable handling of Trends Earth outputs documentation as all outputs will be following the STAC items specification.

Proposed structure

catalog/
│── catalog.json  # STAC root catalog
│── countries/
│   ├── country/  # Country 1 datasets
│   │   ├── country.json  # STAC Collection for country
│   │   ├── country_sdg_15_3_1.json  # STAC Item for country
│   │   ├── assets/
│   │   │   ├── sdg-15-3-1-country-jrc.json
│   │   │   ├── sdg-15-3-1-country-trends.json
│   │   │   ├── sdg-15-3-1-summary.xlsx
│   │   │   ├── sdg-15-3-1-summary.vrt
│   │   │   ├── sdg-15-3-1-baseline.tif
│   │   │   ├── sdg-15-3-1-progress.tif
│── summaries/
│   ├── sdg-15-3-1-summary_band_key.json
│   ├── sdg-15-3-1-summary_summary.json