AWS::EFS::FileSystem - DNSName accesible via Fn::GetAtt

## 1. Title
AWS::EFS::FileSystem - DNSName accesible via Fn::GetAtt

## 2. Scope of request
Make the DNSName accessible using Fn::GetAtt on an AWS::EFS::FileSystem resource.

## 3. Expected behavior
Get back the DNS name created for the AWS::EFS::FileSystem resource, e.g. `fs-XXXXX.efs.us-west-2.amazonaws.com`

## 4. Suggest specific test cases
N/A

## 5. Helpful Links to speed up research and evaluation
N/A

## 6. Category
Storage

## 7. Any additional context
We want to mount EFS automatically in our EC2 instances, so we need to know how to access the it. The EFS file system (and a bunch of MountPoints but those are out of scope here) is created within the same stack. As we can't query the DNS name directly we have to use workarounds like this (it's a snippet from a LaunchTemplate, we're using tags to pass this information but the issue is the same if someone wants to put this into the UserData):
```json
"TagSpecifications": [
{
"ResourceType": "instance",
"Tags": [
{
"Key": "my_company:local:efs_server",
"Value": {
"Fn::Sub": [
"${EFSServer}.efs.${AWS::Region}.amazonaws.com",
{ "EFSServer": { "Ref": "FileSystem" } }
],
}
}
]
}
]
```
Instead of assembling the DNS name manually - and hoping that the generation method won't change without any notice - we would like to use something like this:
```json
"TagSpecifications": [
{
"ResourceType": "instance",
"Tags": [
{
"Key": "my_company:local:efs_server",
"Value": { "Fn::GetAtt": [ "FileSystem", "DNSName" ] }
}
]
}
]
```

Start with the AWS::EFS::FileSystem DNSName request and the Fn::GetAtt example in this issue, then verify the expected regional DNS-name format. Done means the resource exposes the generated DNS name through Fn::GetAtt and the requested use case no longer requires assembling it manually.

Written by the indexing model from the issue text.