First implementation of the oscar_python client

[jzvolensky]

process json PR: eodcgmbh/openeo-processes#46

adds oscar_client and minio as dependencies

implements the oscar_python client library to interact with an OSCAR service

There will probably be a bunch of small changes and fixes here in there as it is quite extensive

CC @clausmichele

@SerRichard is the current _get_refresh_token enough for what we discussed about reading the token from an env variable

[clausmichele]

From what I remember of our discussion, the token should be handled by the openEO back-end and we should "just" read it from an environment variable. Please check my review of the proposed process definition.