AWS::AppSync::GraphQLApi - return API domain output property

## Instructions for CloudFormation Coverage New Issues Template

## 2. Scope of request

`AWS::AppSync::GraphQLApi` should return a value of an assigned domain so it could be possible to set up **CloudFront** distribution origin.

## 3. Expected behavior
## 4. Suggest specific test cases

```yaml
Outputs:
ApiDomain:
Value: !GetAtt "AppSyncApi.DomainName"
```

## 5. Helpful Links to speed up research and evaluation

Samples:
#### https://aws.amazon.com/blogs/mobile/use-a-custom-domain-with-aws-appsync-amazon-cloudfront-and-amazon-route-53/

## 6. Category (required) - Will help with tagging and be easier to find by other users to +1

Use the categories as displayed in the AWS Management Console (simplified):

11. Other (AppSync)