Add Retry-After Header Support

[AryanSharma48]

Many APIs return a Retry-After header when responding with HTTP 429 Too Many Requests.

SmoothAPI currently applies exponential backoff for retries, but does not respect server-provided retry instructions.

This feature should parse and honor the Retry-After header whenever available.

Example

HTTP/1.1 429 Too Many Requests
Retry-After: 30

SmoothAPI should wait 30 seconds before retrying instead of using the calculated backoff delay.

Acceptance Criteria

• Parse the Retry-After header
• Support numeric values
• Fall back to existing backoff behavior when the header is missing
• Add test coverage
• Update documentation if required

Notes

This change should preserve existing behavior when no Retry-After header is present