Ensure data encryption is enabled for AWS SageMaker Notebook instances

AWS SageMaker Notebook instances do not have data encryption enabled which may cause exposure of sensitive data.

In AWS Console -

1. Sign in to the AWS Console and go to SageMaker dashboard.
2. Create a notebook instance.
3. Make sure that permissions and encryption key is selected.
4. Complete the rest of the configurations.

In Terraform -

1. In the aws_sagemaker_notebook_instance resource, set the kms_key_id field to a valid KMS key.

References:
https://docs.aws.amazon.com/sagemaker/latest/dg/encryption-at-rest.html
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/sagemaker_notebook_instance#kms_key_id