Implement an API endpoint that returns relevant versions

[rokroskar]

For many reasons, knowing the version of a component in a deployment is useful and should not require one to access the kubernetes API or have knowledge of the deployment/instance details. One example: automatically form links in the UI pointing to the correct versions of the documentation.

The endpoint should be implemented in a lightweight service living in the main renku repo and it should group together versions from other services. Each service should expose a public /version endpoint that details the relevant versions of the service (there may be multiple). The combined versions should be available under <renku-domain>/api/version.