CloudFormation/CDK Custom Domain Support For AppRunner

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**Tell us about your request**
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.

This should support the creation of relevant constructs in the CDK (https://github.com/aws/aws-cdk/issues/19740)

**Describe alternatives you've considered**
Fargate/ECS

**Additional context**

Dupe of https://github.com/aws-cloudformation/cloudformation-coverage-roadmap/issues/1092

**Attachments**
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