Allow advertising artifacts capabilities

For PR https://github.com/Open-EO/openeo-python-client/pull/750 it would be beneficial if the Openeo Python client can discover the artifacts capabilities.

If there is an artifacts capabilities endpoint it could expose a document like:

 {
  "providers": [
    {
      "id": "s3",
      "type": "s3-sts"
      "cfg": {
	      "s3_endpoint": "https://s3.waw3-1.openeo.v1.dataspace.copernicus.eu",
	      "sts_endpont": "https://sts.waw3-1.openeo.v1.dataspace.copernicus.eu",
	      "buckets": [
	      	{
	      	  "bucket_name": "openeo-artifacts",
	      	  "role": "arn:aws:iam::000000000000:role/S3Access"
	      	},
	      	...
	      ]
      }
    },
    ...
  ]
}

The first provider is the default and the first bucket is also the default bucket.
With this configuration data the client could configure itself to upload artifacts to
a location that is accessible by OpenEO.