refactor(loadbalancer): implement sticky sessions and improve algorithm separation

- Refactor load balancer interface to separate server selection (ChooseServer) from request handling
- Add cookie-based sticky session support with configurable max-age and secure cookie handling
- Integrate idlewatcher requests with automatic sticky session assignment
- Improve algorithm implementations:
  * Replace fnv with xxhash3 for better performance in IP hash and server keys
  * Add proper bounds checking and error handling in all algorithms
  * Separate concerns between server selection and request processing
- Add Sticky and StickyMaxAge fields to LoadBalancerConfig
- Create dedicated sticky.go for session management utilities

internal/net/gphttp/loadbalancer/sticky.go

@@ -0,0 +1,50 @@
+package loadbalancer
+
+import (
+	"encoding/hex"
+	"net/http"
+	"time"
+	"unsafe"
+
+	"github.com/bytedance/gopkg/util/xxhash3"
+	"github.com/yusing/godoxy/internal/types"
+)
+
+func hashServerKey(key string) string {
+	h := xxhash3.HashString(key)
+	as8bytes := *(*[8]byte)(unsafe.Pointer(&h))
+	return hex.EncodeToString(as8bytes[:])
+}
+
+// getStickyServer extracts the sticky session cookie and returns the corresponding server
+func getStickyServer(r *http.Request, srvs []types.LoadBalancerServer) types.LoadBalancerServer {
+	cookie, err := r.Cookie("godoxy_lb_sticky")
+	if err != nil {
+		return nil
+	}
+
+	serverKeyHash := cookie.Value
+	for _, srv := range srvs {
+		if hashServerKey(srv.Key()) == serverKeyHash {
+			return srv
+		}
+	}
+	return nil
+}
+
+// setStickyCookie sets a cookie to maintain sticky session with a specific server
+func setStickyCookie(rw http.ResponseWriter, r *http.Request, srv types.LoadBalancerServer, maxAge time.Duration) {
+	http.SetCookie(rw, &http.Cookie{
+		Name:     "godoxy_lb_sticky",
+		Value:    hashServerKey(srv.Key()),
+		Path:     "/",
+		MaxAge:   int(maxAge.Seconds()),
+		SameSite: http.SameSiteLaxMode,
+		HttpOnly: true,
+		Secure:   isSecure(r),
+	})
+}
+
+func isSecure(r *http.Request) bool {
+	return r.TLS != nil || r.Header.Get("X-Forwarded-Proto") == "https"
+}