Add support for the STAC API Aggregation Extension

[gadomski]

https://github.com/stac-api-extensions/aggregation

Implemented in stac-server, see:

• Per-collection advertisement of available aggregations: https://earth-search.aws.element84.com/v1/collections/sentinel-2-l2a/aggregations
• Requesting that aggregation: https://earth-search.aws.element84.com/v1/collections/sentinel-2-l2a/aggregate?aggregations=total_count

[jonhealy1]

Hi. Can we close this?