Option to set SharedStorageType: Efs encrypted=True

I'm not going to paste all the config that's requested in the Issue template. If that's a problem, please let me know- it's a lot of work to gather all that config info and it's not relevant to this Issue.

We tried to enable SharedStorageType: Efs and create a new cluster, the CloudFormation fails (and rolls back).
In this case, I debugged the problem: our privileged user cannot create *unencrypted* Efs volumes, and the code to generate the CF stack looks like this:
https://github.com/aws/aws-parallelcluster/blob/develop/cli/src/pcluster/templates/cluster_stack.py#L269
```

internal_efs_storage_shared = SharedEfs(
mount_dir="/opt/parallelcluster/init_shared", name="internal_pcluster_shared", throughput_mode="elastic"
)

```

By adding "encrypted=True", I was able to create a cluster with SharedStorageType: Efs

The feature request here is to add some sort of ability to set options for the SharedStorageType: Efs including encryption, as well as the other standard Efs options. This would probably match the existing support for these options in SharedStorage EFS Settings (https://docs.aws.amazon.com/parallelcluster/latest/ug/SharedStorage-v3.html)

Start in cli/src/pcluster/templates/cluster_stack.py around line 269, where the internal SharedEfs is created, and compare the existing SharedStorage EFS Settings documented in the linked guide. Define how SharedStorageType: Efs should expose encryption and other standard EFS options, then verify a generated CloudFormation stack can create the requested encrypted storage.

Written by the indexing model from the issue text.