integration tests for openeo-python-client

[soxofaan]

Idea brought up by @pvbouwel while discussing #805 and #750:

Set up integration tests suite for aspects of python client where unit tests require so much tricky mocking that it becomes almost pointless.

possible features that are good candidates for integration tests:

• OIDC auth flows with (e.g. against dummy keycloak instance)
• multi job manager
• S3 asset management