Back/forwards compatible privileges

[snazy]

Describe the bug

polaris-management-service.yml defines all privileges as enum (values). This approach gets in the way when new privileges are added (client & server are built from different versions of polaris-management-service.yml).

The enum-approach is also not going to work with the short/mid-term goal to have "pluggable authZ" in Polaris, where authZ implementations can define their own privileges.