[Alerting] Update alerting feature privilege to allow subfeature privilege separation

[ymao1]

As part of this issue, we have refactored the alerts authorization client to provide a generic consumer/producer based RBAC authorization on alerting entities (currently rule and alert). In order to support the subfeature privilege model, where for example, users may have read privileges on Alerting (Rules & Alerts) but be given elevated subfeature privilege to update alerts, we need to update the alerting feature privilege schema to handle separated feature strings for rules and alerts.

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)