Allow advertising artifacts capabilities

[pvbouwel]

For PR Open-EO/openeo-python-client#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.

[soxofaan]

I added with Open-EO/openeo-geopyspark-driver@ae6bc3b a quick, simple but generic capabilities_extras config to easily append to the general capabilities document. Should serve as initial solution for this feature. When things are more settled in terms of expected schema we can upgrade this to a dedicated endpoint and/or API extension when necessary