Back/forwards compatible privileges

### 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.

Read polaris-management-service.yml first to understand how privileges are currently represented and how client/server version differences arise. Done means defining a compatibility approach that permits newly added and pluggable privileges without enum-version breakage; the issue names no tests or entry points.

Written by the indexing model from the issue text.