Split AWS.RDS.Types into several modules.
Add RDS/DescribeDBSubnetGroups.
Add a test case for RDS/DescribeDBSubnetGroups.
Add RDS/DescribeDBParameterGroups.
Add a test case for RDS/DescribeDBParameterGroups.
Add RDS/DescribeDBSecurityGroups.
Add a test case for RDS/DescribeDBSecurityGroups.