[ECR] [request]: Multi-region endpoints for ECR registries

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**Tell us about your request**
To combine multiple regional registries to form a single "virtual" registry, automatically doing failover and performance-aware routing.

```
# routes to us-east-1 or ap-southeast-2 depending on what's available and where I'm calling from
docker pull organization-name.ecr.amazonaws.com/repo
```

This should work even if I use different accounts for different regions.

**Which service(s) is this request for?**
ECR

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
Disaster recovery automation. Speeding up registry access.

**Are you currently working around this issue?**
Tooling to get orchestrators to point to a different regional registry in case of disaster.