Ensure the CloudWatch Log Group is encrypted by KMS/CMK

Create the possibility to encrypt the VCP Flow Logs Cloudwatch Log Group using KMS Customer Managed Keys (CMK)

Types of changes

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[ ] Bugfix (non-breaking change which fixes an issue)
[X] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[ ] Documentation Update (if none of the other choices apply)

Checklist

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[ ] I have read the CONTRIBUTING.md doc.
[X] I have added necessary documentation (if appropriate).
[ ] Any dependent changes have been merged and published in downstream modules.

Further comments

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DNXLabs / terraform-aws-network