Multiple reserved feature privileges

[kobelb]

Currently, we support a single reserved feature privilege. This creates a Kibana Privilege which we then associated with reserved roles for both Machine Learning and Monitoring. This allows us to consume these Kibana Privileges within Kibana to hide/show the applications, catalogue entries, etc. in a consistent manner.

This reserved privilege is currently singular, so both the machine_learning_user and the machine_learning_admin get the same reserved_ml privileges. This will become prohibitive as ML migrates their code to prepare for space-awareness, as they can't rely consistently on UI Capabilities to do things like hide the create ML job button. If we allow them to register multiple reserved privileges, this will allow them to do so.

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)

[kobelb]

/cc @droberts195 @jgowdyelastic @peteharverson

[legrego]

Resolved via #61980