[AWS::RDS::DBInstance] - [Enhancement] - Implement `GetAtt` for the `DB instance name`

[r-heimann]

See aws-cloudformation/cloudformation-coverage-roadmap#1625
Should be fairly easy to implement, since it already exists as a !Ref.

[zrfr]

!GetAtt DBInstance.DBInstanceIdentifier already works today.

Note that this is not documented and is not officially supported by the CFN framework. To officially support and document this we are blocked on aws-cloudformation/cloudformation-coverage-roadmap#68.