[CustomDbEngineVersion] Add new parameters in CEV

[0ooopsFibby]

Issue #, if available: N / A

Description of changes:

Latest AWS RDS SDK has added three optional parameters for CEV model and originally required field is now no longer compulsory. This PR is to add the additional parameters to CFN accordingly.

• unit tests doesn't require changes as it was generated based on model
• contract tests have passed locally

  curl -XPOST "http://localhost:9000/2015-03-31/functions/function/invocations" -d '{ "TypeName": "AWS::RDS::CustomDBEngineVersion", "Bucket": "jiayuwa-cfn", "Key": "aws-service-resource-cev.zip", "TestDetails":["TestCreate,test_create_list", "TestCreate,test_create_create", "TestRead,test_governance", "TestRead,test_read_return_write_only_properties", "TestRead,test_read_return_readonly_properties", "TestDelete,test_delete_read", "TestDelete,test_delete_delete", "TestDelete,test_delete_list", "TestUpdate,test_update_read", "TestUpdate,test_update_list", "TestRead,test_read_return_all_schema_properties_using_one_or_more_inputs"] }' 

  test_read_input_output_negative_match from contract test doesn't apply for the API, this is because some parameters are not supposed to be used together (e.g. S3 bucket and source CEV)

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