Improve partial user job listing

[soxofaan]

Originates from https://github.com/openEOPlatform/architecture-docs/issues/179:

When a back-end is (temporarily) down or long response time, it could happen that the user jobs listing (GET /jobs) of the aggregator skips a non-responsive back-end, resulting in partial user job listing. While this is better than completely failing, it can be confusing to end user if it is not clear that result is partial.

Ideas to improve:

• HTTP header that flags response as incomplete in some way Generic way for warnings/deprecations on API response openeo-api#412
• "federation:missing" flag from federation extension Federation Extension openeo-api#419

[soxofaan]

Added initial support for "federation:missing" in aggregator and python client
also see https://github.com/openEOPlatform/architecture-docs/issues/179#issuecomment-1007453499