feat: add rate limiting retry middleware

[adityakrmishra]

Addresses #45

Hi team,

This PR introduces an opt-in middleware to automatically wait and retry requests when hitting GitHub's rate limits or secondary abuse limits (HTTP 403), as requested in #45.

To address the concerns about indefinite blocking and race conditions, this feature is completely opt-in and requires a strict maxWait threshold.

Implementation Details:

• Opt-in only: Added WithRateLimitRetry(maxWait) as a ClientOption.
• Configuration: Added RateLimitMaxWait to Config (configurable via YAML/JSON or GITHUB_RATE_LIMIT_MAX_WAIT env var).
• Header Parsing: Respects both Retry-After (seconds) and X-RateLimit-Reset (epoch timestamp), prioritizing Retry-After as per GitHub API docs.
• Safety First: - If the required wait exceeds maxWait, it returns the 403 response immediately.
  • Retries exactly once after sleeping.
  • Sleep is context-aware (select { case <-ctx.Done() ... }) to prevent goroutine leaks if the caller cancels the request.
  • Safely drains and closes the original 403 response body before retrying to prevent connection exhaustion.

Testing:
Added comprehensive table-driven tests in middleware_retry_test.go covering both header types, edge cases (wait > maxWait, no headers), and zero-config scenarios.

Let me know if you would like any changes to the default behavior or parsing logic!

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feat: add rate limiting retry middleware

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