Create a API for listing the available audit event types

Description

We've created a couple of audit event type YMLs that are defined under ee/config/audit_events/types/<event-type-name>.yml and config/audit_events/types/<event-type-name>.yml. We should create an API that lists the audit event type from this YML that can be used to show audit events and search for audit event streaming filters filter streaming audit events based on event type.

Proposal