provider/aws: Add dns_name to aws_efs_mount_target

[radeksimko]

This can be useful for people who prefer DNS names over IPs in configs.

It also expands the permissions scope for aws_efs_mount_target as ec2:DescribeSubnets is now required. I think it's worth expanding, given the convenience.

Test plan

make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSEFSMountTarget'

TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSEFSMountTarget -timeout 120m
=== RUN   TestAccAWSEFSMountTarget_basic
--- PASS: TestAccAWSEFSMountTarget_basic (416.17s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    416.193s

[stack72]

LGTM! Merge at will :)

[ghost]

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