Ensure inter-cluster encryption is enabled for AWS MSK cluster

Not using inter-cluster encryption for AWS MSK clusters can impact the confidentiality of the data in transit.

Encryption settings are configured with TLS 1.2 by default, however this can be overridden at the time a cluster is created. To learn more, see the AWS documentation (below).

In Terraform -

1. In the aws_msk_cluster resource, configure an encryption_info block.
2. Within the encryption_info block, create an encryption_in_transit block with client_broker set to TLS and in_cluster set to true.

References:
https://docs.aws.amazon.com/msk/latest/developerguide/msk-encryption.html
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/msk_cluster#encryption_in_transit