Add automatic retry support for 429 responses

[dremin]

Issue Summary

When working in large production environments, it is quite easy to run into scenarios where the Twilio API returns a 429 due to rate limiting. The Node SDK added support for automatic retry with exponential backoff in twilio/twilio-node#886. It would be great if the Go SDK could add similar functionality.

[tiwarishubham635]

Hi @dremi! Thank you for the heads up! This issue has been added to our internal backlog to be prioritised. Pull requests and +1s on the issue summary will help it move up the backlog.
Thanks!