Support certificate from ACM to simplify running behind CloudFront in a multi-region setup

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**Tell us about your request**

I have been successfully using AppRunner in a single region for more than a year and it's working fine. Now I am trying to distribute the service globally to decrease service latency.

My initial approach was to create identical AppRunner services in multiple regions, and then use Route 53's latency-based routing and use that hostname as the origin for a CloudFront distribution.

The problem appears to be that if the CloudFront distribution is available under `api.example.com`, and the latency-based record for the origin is `api-origin.example.com`, then the setup won't work because of SSL certificate validation. I can not associate an AppRunner service with a certificate for `api-origin.example.com`, I can only do "Link Domain" which fails because in addition to creating a certificate it also wants to create a CNAME – which collides with the latency-based record.

Being able to use a certificate from ACM would solve this.

**Describe alternatives you've considered**

I've considered separate origins for each AppRunner service and having a Lambda@Edge function look up a latency based TXT containing the actual origin hostname, then override the origin request's `Host` header. I have not tried if this actually works, and I would like to avoid the overhead of this approach is it is significantly more complex.

**Additional context**

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