Add retry transport with configurable backoff and Retry-After support

Implements retry middleware as a RoundTripper wrapper:
- Exponential and constant backoff strategies with jitter
- RFC 7231 Retry-After header parsing (seconds and HTTP-date)
- Default policy retries idempotent methods on 429/5xx and network errors
- Body restoration via GetBody, context cancellation, response body cleanup

retry/retry_after.go

@@ -0,0 +1,43 @@
+package retry
+
+import (
+	"net/http"
+	"strconv"
+	"time"
+)
+
+// ParseRetryAfter extracts the delay from a Retry-After header (RFC 7231).
+// It supports both the delay-seconds format ("120") and the HTTP-date format
+// ("Fri, 31 Dec 1999 23:59:59 GMT"). Returns the duration and true if the
+// header was present and valid; otherwise returns 0 and false.
+func ParseRetryAfter(resp *http.Response) (time.Duration, bool) {
+	if resp == nil {
+		return 0, false
+	}
+
+	val := resp.Header.Get("Retry-After")
+	if val == "" {
+		return 0, false
+	}
+
+	// Try delay-seconds first (most common).
+	if seconds, err := strconv.ParseInt(val, 10, 64); err == nil {
+		if seconds < 0 {
+			return 0, false
+		}
+		return time.Duration(seconds) * time.Second, true
+	}
+
+	// Try HTTP-date format (RFC 7231 section 7.1.1.1).
+	t, err := http.ParseTime(val)
+	if err != nil {
+		return 0, false
+	}
+
+	delay := time.Until(t)
+	if delay < 0 {
+		// The date is in the past; no need to wait.
+		return 0, true
+	}
+	return delay, true
+}