feat(api-rest): add rest default auth mode

[soberm]

Description of changes

Adds support for the defaultAuthMode parameter in REST API operations, allowing
users to control the default authentication behavior. More specifically:

• Added defaultAuthMode?: RESTAuthMode to REST API method options
• Added RESTAuthMode type definition ('none' | 'iam')
• Updated REST client to handle authentication mode

• Added global configuration support via libraryOptions.API.REST.defaultAuthMode
• Added parameter passing from public API through to REST calls
• Added validation for defaultAuthMode parameter handling
• Updated tests for new functionality

Usage

Per-request configuration:

await get({
  apiName: 'myApi', 
  path: '/public-endpoint',
  options: {
    defaultAuthMode: 'none' // Skip default IAM authentication for this request
  }
});

Global configuration:

Amplify.configure({
  // ... other config
}, {
  API: {
    REST: {
      defaultAuthMode: 'none' // Default mode for all REST calls
    }
  }
});

Issue #, if available

#14460

Description of how you validated changes

Unit tests

Checklist

• PR description included
• yarn test passes
• Unit Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)

Checklist for repo maintainers

• Verify E2E tests for existing workflows are working as expected or add E2E tests for newly added workflows
• New source file paths included in this PR have been added to CODEOWNERS, if appropriate

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.