Add a /health endpoint to Federation Extension

[m-mohr]

In openEO Platform there's a GET /health endpoint used by the aggregator. It would be useful for the federation extension, I think.
It looks like there's no standardized format for the response yet.

Examples:

• https://openeo.eodc.eu/openeo/1.1.0/health
• https://openeo.vito.be/openeo/1.1/health
• https://jjdxlu8vwl.execute-api.eu-central-1.amazonaws.com/production/health

[soxofaan]

FYI: the aggregator itself also responds to /health:
https://openeocloud.vito.be/openeo/1.1/health

It aggregates the HTTP status code and bodies (if any) of upstream backends: also see https://github.com/Open-EO/openeo-aggregator/blob/e17e9beefb0b787b6be7a97d42beed6a9572822e/src/openeo_aggregator/backend.py#L1357-L1385

[m-mohr]

By the way, why did we introduce a new endpoint instead of just checking GET /?

[soxofaan]

the idea is to do a bit more than just returning a somewhat static capabilities doc. E.g. in geopyspark implementation we do a quick spark operation to verify that the spark basics are working. In the aggregator we also explicitly loop over all upstream back-ends and measure response time.

I think an important aspect here is caching: the capabilities doc is usually very static, so you'll allow heavy caching on that, while a "health" check should be as up to date as possible with as little as caching as possible.

[m-mohr]

Makes sense, thanks.