(feat) Cross-region backup support

[kurtismash]

Allow the module caller to specify backup_regions (or something similar) for each Deployment, this will be a list of regions that backups are copied to before reaching the immutable (Standard/LAG) Vaults. There are some resource types that do not support cross-account and cross-region copies in a single action, these will need to be considered. It is also possible that the EventBridge architecture must be changed to enable Step Functions to run in the destination regions.

[kurtismash]

Amazon RDS announced cross-Region and cross-account snapshot copy yesterday, I assume this will reduce the complexity of this feature.

The AWS Backup docs haven't yet been updated, still including "³ RDS, Aurora, DocumentDB, and Neptune do not support a single copy action that performs both cross-Region AND cross-account backup. You can choose one or the other.". There is no mention of DocumentDB in this announcement, only RDS and Aurora.

[kurtismash]

AWS Backup now supports cross-region, cross-account copies in a single action for Amazon RDS, Amazon Aurora, Amazon Neptune, and Amazon DocumentDB snapshots.