Pre-create role used by "Subdomain auto-detection"

**Is your feature request related to a problem? Please describe.**

Cannot enable "Subdomain auto-detection" for a custom domain.

When editing a custom domain, there is an option to enable "Subdomain auto-detection." Checking that box, it exposes an input window with a bit of tiny text below it: "Amplify will create a role in your account to access and update your Route53 hosted zone."

However, our IAM policies require attaching a Permissions Boundary to any role we create. Since Amplify Console is unaware of that requirement, the role creation fails when enabling "Subdomain auto-detection". (Or rather, when you click "Update" it just spins and spins with no indication of failure, but it never succeeds.)

**Describe the solution you'd like**

Enough details about the role and the required permissions to create it in advance. I assume it is looking for a role with a specific name?

Start with the custom-domain editor's “Subdomain auto-detection” option and the Update action described in the issue, then trace how Amplify creates the Route53 access role. Confirm the required IAM permissions and Permissions Boundary behavior; done means administrators can prepare or use a compatible role and failed creation no longer appears to spin indefinitely.

Written by the indexing model from the issue text.