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/options.go

@@ -0,0 +1,56 @@
+package retry
+
+import "time"
+
+type options struct {
+	maxAttempts int     // default 3
+	backoff     Backoff // default ExponentialBackoff(100ms, 5s, true)
+	policy      Policy  // default: defaultPolicy (retry on 5xx and network errors)
+	retryAfter  bool    // default true, respect Retry-After header
+}
+
+// Option configures the retry transport.
+type Option func(*options)
+
+func defaults() options {
+	return options{
+		maxAttempts: 3,
+		backoff:     ExponentialBackoff(100*time.Millisecond, 5*time.Second, true),
+		policy:      defaultPolicy{},
+		retryAfter:  true,
+	}
+}
+
+// WithMaxAttempts sets the maximum number of attempts (including the first).
+// Values less than 1 are treated as 1 (no retries).
+func WithMaxAttempts(n int) Option {
+	return func(o *options) {
+		if n < 1 {
+			n = 1
+		}
+		o.maxAttempts = n
+	}
+}
+
+// WithBackoff sets the backoff strategy used to compute delays between retries.
+func WithBackoff(b Backoff) Option {
+	return func(o *options) {
+		o.backoff = b
+	}
+}
+
+// WithPolicy sets the retry policy that decides whether to retry a request.
+func WithPolicy(p Policy) Option {
+	return func(o *options) {
+		o.policy = p
+	}
+}
+
+// WithRetryAfter controls whether the Retry-After response header is respected.
+// When enabled and present, the Retry-After delay is used if it exceeds the
+// backoff delay.
+func WithRetryAfter(enable bool) Option {
+	return func(o *options) {
+		o.retryAfter = enable
+	}
+}