fix(data): fix library configuration options headers for subscriptions (#12590)

[david-mcafee]

Description of changes

Passes headers to the AWSAppSyncRealTimeProvider when calling Amplify.configure. For example:

import { Amplify } from "aws-amplify";
import config from "./amplifyconfiguration.json";

Amplify.configure(config, {
    API: {
        GraphQL: {
            headers: async () => ({
                Authorization: 'example-token'
            }),
        },
    },
});

headers here was previously graphql_headers in V5. These are separate from custom headers, and will always take precedence over custom headers (i.e. the Authorization token will never be overwritten).

Issue #, if available

#12590

Description of how you validated changes

Checklist

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

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

[elorzafe]

Looks good

thanks @david-mcafee 🎖️

[kvramyasri7]

Looks good

thanks @david-mcafee 🎖️🥇🎖️