AWS App Runner Custom Domains

### Name of the resource

Other

### Resource name

AWS::AppRunner::CustomDomain

### Description

There is no CloudFormation support for creating custom domains for AWS App Runner. There should be a new resource created called something like AWS::AppRunner::CustomDomain. The related links included have the fields that need to be included.

### Other Details

https://docs.aws.amazon.com/apprunner/latest/api/API_AssociateCustomDomain.html
https://docs.aws.amazon.com/apprunner/latest/api/API_DescribeCustomDomains.html
https://docs.aws.amazon.com/apprunner/latest/api/API_DisassociateCustomDomain.html

Start with the linked AWS App Runner AssociateCustomDomain, DescribeCustomDomains, and DisassociateCustomDomain API documentation, then determine the CloudFormation resource shape for AWS::AppRunner::CustomDomain. Done means the resource supports the fields and lifecycle operations required to create, inspect, and remove an App Runner custom domain.

Written by the indexing model from the issue text.