resource ApiGatewayCustomDomainMapping does not support attribute type Arn in Fn::GetAtt

I'm wondering if anybody else has seen this error. I decided to make a acm cert, in cdk and reference if like so in the chalice stage config.

```
cdk: 1.132.0 (build 5c75891)
chalice 1.26.0, python 3.9.4, linux 5.11.0-38-generic
```

This is the error, config will follow.
``` ❌ myapp-chalice-api failed: Error: Failed to create ChangeSet cdk-deploy-change-set on myapp-chalice-api: FAILED, Template error: resource ApiGatewayCustomDomainMapping does not support attribute type Arn in Fn::GetAtt
at /home/project_dir/myapp/node_modules/aws-cdk/lib/api/util/cloudformation.ts:229:11
at processTicksAndRejections (internal/process/task_queues.js:97:5)
at waitFor (/home/project_dir/myapp/node_modules/aws-cdk/lib/api/util/cloudformation.ts:190:20)
at Object.waitForChangeSet (/home/project_dir/myapp/node_modules/aws-cdk/lib/api/util/cloudformation.ts:215:15)
at prepareAndExecuteChangeSet (/home/project_dir/myapp/node_modules/aws-cdk/lib/api/deploy-stack.ts:294:32)
at CdkToolkit.deploy (/home/project_dir/myapp/node_modules/aws-cdk/lib/cdk-toolkit.ts:194:24)
at initCommandLine (/home/project_dir/myapp/node_modules/aws-cdk/bin/cdk.ts:267:9)
Failed to create ChangeSet cdk-deploy-change-set on myapp-chalice-api: FAILED, Template error: resource ApiGatewayCustomDomainMapping does not support attribute type Arn in Fn::GetAtt
```

api cert setup

```
self.api_cert = acm.DnsValidatedCertificate(
self,
"apicert",
domain_name=self.api_fqdn,
hosted_zone=self.domain,
)
```

This created just fine.

and in the config dict.

```
...
"api_gateway_custom_domain": {
"domain_name": self.api_fqdn,
"certificate_arn": self.api_cert.certificate_arn,
},
...
```

removing this from the config makes the error go away.

Oh, I also put the cert arn as a static string, i.e. `arn:13234:...certificate` and it threw the same error.

Cheers, and thank you for the wonderful project.

If anybody has a word around, I'd love to hear it.

Start by reproducing the failure from the Chalice stage configuration using the api_gateway_custom_domain certificate_arn reference, then inspect the generated CloudFormation and the deployment path mentioned in aws-cdk/lib/api/util/cloudformation.ts and deploy-stack.ts. Done means the generated template no longer requests the unsupported Arn attribute and the stack creates its change set successfully.

Written by the indexing model from the issue text.