Add automatic retry with exponential backoff for 429 responses

[Copilot]

Large-scale Twilio API usage frequently encounters 429 rate-limit responses. The Go SDK had no retry mechanism, requiring callers to implement their own backoff logic.

Changes

• Client.MaxRetries int — new opt-in field (default 0 = disabled, backward compatible); set to a positive value to enable automatic retries on 429
• retryAfterDuration — computes the wait before each retry: honours the Retry-After response header (numeric seconds and HTTP-date/RFC 7231 formats), falling back to exponential backoff (500 ms → 1 s → 2 s … capped at 64 s)
• doWithErr — retries the request up to MaxRetries times when the server returns HTTP 429, resetting the request body via GetBody between attempts
• SendRequest — sets req.GetBody on POST/PUT/PATCH and JSON requests so the body can be replayed on retry

Usage

client := twilio.NewRestClientWithParams(twilio.ClientParams{
    Username: accountSid,
    Password: authToken,
})
client.SetMaxRetries(3) // retry up to 3 times on 429

Or directly:

client.Client.MaxRetries = 3

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add automatic retry support for 429 responses</issue_title>
<issue_description>### 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.
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add automatic retry support for 429 responses #247

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