Improve support for the Federation Extension

[m-mohr]

In the EOEPCA+ project, we made an evaluation of the federation-related functionality available through the openEO Julia client.
The Federation Extension can be found here: https://github.com/Open-EO/openeo-api/blob/draft/extensions/federation/README.md

The following issues have been identified:

• The federation property in GET / is not available, which is due to Retrieval of capabilities #31
• federation:missing can't be accessed through the corresponding functions for the following endpoints:
  • GET /collections
  • GET /processes
  • GET /jobs

Additional functionality might be missing in the future for other endpoints which have not been implemented yet (e.g. GET /file_formats as reported in #30)

In addition the following general issue has been identified: #29

[danlooo]

Thanks for the suggestion! When can we expect to have a stable API for the Federation Extension?

[m-mohr]

It's a work in progress, but more towards additions. What is already in the document should be pretty stable. I don't expect significant changes there.

Also, for context: I did this evaluation for another project to see how far each client is with regards to federations. We don't expect any work on the client right now to support it. This was more a status report :-)