AWS::EC2::TrafficMirrorTarget - Arn accessible via Fn::GetAtt

## 1. Title
AWS::EC2::TrafficMirrorTarget - Arn accessible via Fn::GetAtt

## 2. Scope of request
We want to share a TrafficMirrorTarget between our accounts. The AWS::RAM::ResourceShare accepts only ARNs so we have to assemble the ARN of the TrafficMirrorTarget manually (and hoping that the template won't change without any notice in the future).
It's a snippet from an AWS::RAM::ResourceShare resource.
```json
"ResourceArns": [
{
"Fn::Sub": [
"arn:aws:ec2:${AWS::Region}:${AWS::AccountId}:traffic-mirror-target/${MirrorTargetId}",
{ "MirrorTargetId": { "Ref": "MirrorTarget" } }
]
}
]
```
It would be much more easier - and future proof - if we can fetch the ARN directly.
```json
"ResourceArns": [
{ "Fn::GetAtt": [ "MirrorTarget", "Arn" ] }
]
```

## 3. Expected behavior
Get back the ARN.

## 4. Suggest specific test cases
N/A

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

## 6. Category
Networking

## 7. Any additional context
N/A