Enable global write forwarding

[dbbh]

Issue #, if available:

Description of changes: This change adds EnableGlobalWriteForwarding to Create and ModifyDBCluster requests.

Supported APIs:

• https://docs.aws.amazon.com/AmazonRDS/latest/APIReference/API_ModifyDBCluster.html
• https://docs.aws.amazon.com/AmazonRDS/latest/APIReference/API_CreateDBCluster.html Non-Supported APIs:
• https://docs.aws.amazon.com/AmazonRDS/latest/APIReference/API_RestoreDBClusterToPointInTime.html
• https://docs.aws.amazon.com/AmazonRDS/latest/APIReference/API_RestoreDBClusterFromSnapshot.html

Added unit tests to ensure the parameter is picked up by the Translator.

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