support `/processes` version

[soxofaan]

related to

• Add openeo.cloud variant with support for 2.0.0 processes openeo-aggregator#149
• Federation extension: how to ensure consistent process version across backends? openeo-api#548
• How to expose which version of openeo-processes is available/targeted  openeo-api#517
• Add openEO processes version #517 openeo-api#549