Add Renku PEP; update contract structure; add KMS; refactor POC directory

This PR brings the following changes:

• Add a Renku PEP that authenticates users with Keycloak and OpenID Connect, and starts sessions on Renku. The PEP can mount S3 buckets on behalf of users
• Update the contract structure to make it more functional: function becomes a "core attribute" of the Verb struct, owner becomes a "core attribute" of the Object struct
• Add a KMS (Key Management Service) that stores users public keys in the database. The KMS is currently very basic and does not provide any authentication mechanism. It is there for a demonstration purpose
• Refactor some of the directories: some services (such as the UCM user interface) have been moved to the POC directory