Context: https://github.com/RedHatInsights/rhsm-subscriptions/pull/3824#discussion_r1785960735

The role is mainly used to identify products. When creating events, we map a Event.Role enum from a string, and when being used, we map back the Event.Role into a string.
The problem is that we need to maintain the enum values with new/updated roles which is not directly managed by us (roles are coming from telemeter/prometheus...).

It would be easier to simply use a string.

Acceptance Criteria

• Remove Event.Role enum to be replaced by string type