refactor(smalltrip,problems): define RetryAfter header parsing into separated type

smalltrip/problem/headers.go

@@ -0,0 +1,33 @@
+package problem
+
+import (
+	"encoding"
+	"fmt"
+	"net/http"
+	"time"
+)
+
+type RetryAfter[T time.Time | time.Duration] struct {
+	time T
+}
+
+var (
+	_ encoding.TextMarshaler = RetryAfter[time.Time]{}
+	_ fmt.Stringer           = RetryAfter[time.Time]{}
+)
+
+func (h RetryAfter[T]) MarshalText() ([]byte, error) {
+	return []byte(h.String()), nil
+}
+
+func (h RetryAfter[T]) String() string {
+	switch t := any(h.time).(type) {
+	case time.Time:
+		if !t.IsZero() {
+			return t.Format(http.TimeFormat)
+		}
+	case time.Duration:
+		return fmt.Sprintf("%.0f", t.Seconds())
+	}
+	return ""
+}