Support GraphQL subscriptions for non-Appsync endpoints

[thinknirmal]

Is your feature request related to a problem? Please describe.
Creating subscription queries through Amplify will eliminate the need to use other libraries like Apollo. This is sub-optimal when we are already using Amplify for authentication.

Describe the solution you'd like
Allow subscriptions to non-Appsync GraphQL endpoints.

Describe alternatives you've considered
Alternative would be to use other libraries like Apollo.

Thank you.