Fix audit logging for `AuthzRolesResource`

[patrickmann]

/nocl CL in related enterprise issue /jpd Graylog2/graylog-plugin-enterprise#12738

Description

Provide proper audit logging when adding / removing users from roles. We were re-using an existing audit event, which did not have the proper signature. This resulted in audit logs with missing information.

Motivation and Context

Relates to Graylog2/graylog-plugin-enterprise#12455

How Has This Been Tested?

Local test

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Refactoring (non-breaking change)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)