Add RDS engine type "oracle-ee-cdb"

[rodolfo-nadai-encora]

I notice that in troposphere/validators/rds.py the VALID_DB_ENGINES list is probably missing oracle-ee-cdb as a possible option.

https://github.com/cloudtools/troposphere/blob/main/troposphere/validators/rds.py#L87

Where I work, we have clients that use that option for deploying Oracle databases on AWS.