* Add filtering support for RDS::DBCluster
* Add support for filtering by `db-cluster-id`.
* Add support for filtering by `engine`.
Obviates the need for the recent change (#6114) because the filter definition maps
`db_cluster_id` to either `db_cluster_identifier` or `db_cluster_arn`.
* No longer required due to filter configuration.
* Account for Neptune weirdness...