Improve support for MBeans without a backing endpoint by introducing
a `JmxEndpoint` interface. The `JmxEndpoint` is intentionally
similar in design to the `MvcEndpoint` from the `mvc` package and
allows for completely custom JMX beans that are not backed by any
real actuator `Endpoint`.

The `AuditEventsMBean` has been refactored to use the new interface and
has been renamed to `AuditEventsJmxEndpoint`.

See gh-6579