Add 'dns_name' attribute to aws_efs_file_system resource

[alewando]

This adds a 'dns_name' attribute to the aws_efs_file_system resource, reflecting the AZ-agnostic naming convention described in AWS docs. A similar change was made to aws_efs_mount_target and this PR mimics that change at the filesystem level. Technically, I think the change to aws_efs_mount_target was incorrect, since the AZ-specific names that terraform was generating for mount targets are still valid and there is now no way to get them. I did not undo that change here, however, since I imagine some users are relying on it. In any case, getting a AZ-agnostic DNS name (useable anywhere in the VPC) seems more natural coming from the filesystem itself than from an AZ-specific mount target.

See hashicorp/terraform#10902 and hashicorp/terraform#11023

[radeksimko]

Thanks, LGTM. I think we can keep the dns_name in mount target - I can see it being useful for users which manage FS separately from MTs and for compatibility reasons.

[ghost]

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