feature: artifact-helper

[pvbouwel]

Allow uploading artifacts using:

from openeo.extra.artifacts import ArtifactHelper

artifact_helper = ArtifactHelper.from_openeo_connection(connection)
storage_uri = artifact_helper.upload_file(object_name, src_file_path)
presigned_uri = artifact_helper.get_presigned_url(storage_uri)

Where object_name is a logical string name that will be used to give a unique name in the backend storage.
And src_file_path points to a local file that needs to be stored remotely.

This MVP version still has the sts and s3 URIs hard coded. This is temporary. It is possible to expand the links that are exposed at the root_url (https://openeo.dataspace.copernicus.eu/openeo/1.2/) to include rels like 'artifacts-s3-endpoint' and 'artifacts-sts-endpoint'. That way different artifact backends can be chosen based on the OpenEO environment.

[soxofaan]

this is quite a large diff 😄 and I haven't gone deep yet, but here are some initial notes

[soxofaan]

I'm a bit short on resources to go deep, so just some superficial notes:

This PR adds a lot of new things (lot of new modules, lot of classes) to the public API of the openeo package. Is it necessary to make all of this public? Once something is public, it's hard to change things because you have to care about compatibility. It's also overwhelming to the user I think.

I also think this feature should be documented in the docs, e.g. somewhere under https://open-eo.github.io/openeo-python-client/cookbook/index.html . Doing that documentation exercise will also make it more clear what the minimal public user-facing part of the API is.
I see you already did some docs on ArtifactHelper, so it should be easy to bootstrap initial docs from that like done at

openeo-python-client/docs/cookbook/job_manager.rst

Lines 13 to 14 in 7e65087

	.. autoclass:: openeo.extra.job_management.MultiBackendJobManager
	:members:

[pvbouwel]

@soxofaan provided docs as requested. I did move around code but mostly renames of files to start with _ to discourage users from entering.

Other changes:

• introduced feature_flags for s3sts such that backend can advertise what is supports
• now decided on advertised feature flag whether to enable/disable checksumming
• more tests
• Added JSON lexer for sphinx as JSON would not render properly otherwise

[pvbouwel]

Unittests are failing because of another change: #788