Amplify auth extension - IAM roles granted to schema

### Is this related to a new or existing framework?

Next.js

### Is this related to a new or existing API?

Authentication

### Is this related to another service?

_No response_

### Describe the feature you'd like to request

Hi! I am not sure if this feature is missing or just not found in docs, but I think it would be very useful if we could define different "authorized" objects - one for UserPool and one for IAM. The idea is to add to the default userpool (front end users) a possibility to define multiple rules for IAM roles (backend services not necessarily related to Amplify)

### Describe the solution you'd like

```
const schema = a
.schema({
Todo: a
.model({
content: a.string(),
})
.authorization((allow) => [
allow.authenticated(),
// plus IAM
allow.iamRoles([env.COPIER_SERVICE_IAM_ROLES]).to(["read"]),
allow.iamRoles([env.UPDATER_SERVICE_IAM_ROLES]).to(["read", "write"]),
]),
```

### Describe alternatives you've considered

Considered altering CF resources, but not sure where to change and what.

### Additional context

_No response_

### Is this something that you'd be interested in working on?

- [ ] 👋 I may be able to implement this feature request
- [ ] ⚠️ This feature might incur a breaking change

No files, tests, or entry points are named; begin by tracing the authentication schema authorization extension and the CloudFormation resources mentioned in the alternatives. Compare the existing UserPool and IAM authorization paths, and consider the requested role-specific read/write rules; done means the schema can express those separate IAM roles alongside authenticated access.

Written by the indexing model from the issue text.