feat(auth): support clientmetadata for token refresh

[soberm]

Description of changes

Adds support for dynamically providing client metadata for Cognito token refresh operations through a ClientMetadataProvider function type. More specifically:

• Added ClientMetadataProvider function type to @aws-amplify/core types
• Added setClientMetadataProvider() method to CognitoUserPoolsTokenProvider
• Integrated ClientMetadataProvider into TokenOrchestrator for token refresh operations
• Added async client metadata provider support to token refresh flow
• Updated tests for new functionality

Usage

// Set global clientMetadata (affects all token refreshes)
import { cognitoUserPoolsTokenProvider } from 'aws-amplify/auth/cognito';

const clientMetadataProvider = () => Promise.resolve({
  'app-version': '1.0.0',
  'device-type': 'mobile'
});

cognitoUserPoolsTokenProvider.setClientMetadataProvider(clientMetadataProvider);

// Per-call clientMetadata (overrides global)
import { fetchAuthSession } from 'aws-amplify/auth';

const session = await fetchAuthSession({
  forceRefresh: true,
  clientMetadata: { 
    'context': 'user-action',
    'operation': 'manual-refresh'
  }
});

// Existing usage remains unchanged (backward compatible)
const session = await fetchAuthSession({ forceRefresh: true });

Issue #, if available

#13379

Description of how you validated changes

• Added unit tests for setClientMetadataProvider functionality

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.