feat(auth): add skipauth metadata option to bypass auth for whitelisted client IPs

Introduces WhitelistedAuthenticator that wraps an auth.Authenticator to skip
authentication when the client IP matches configured CIDR ranges or exact IPs.
The wrapping happens at service-parse time, transparently covering all handler
types (HTTP, SOCKS4/5, relay, http2, SSH).

Usage:
  gost -L 'user:pass@:5999?skipauth=192.168.0.0/24,172.22.22.22/32'

auth/whitelist.go

@@ -0,0 +1,61 @@
+package auth
+
+import (
+	"context"
+	"net"
+	"strings"
+
+	"github.com/go-gost/core/auth"
+	xctx "github.com/go-gost/x/ctx"
+	"github.com/go-gost/x/internal/matcher"
+)
+
+// whitelistedAuthenticator wraps an Authenticator to skip authentication
+// for client IPs matching the configured whitelist patterns.
+type whitelistedAuthenticator struct {
+	auther      auth.Authenticator
+	ipMatcher   matcher.Matcher
+	cidrMatcher matcher.Matcher
+}
+
+// WhitelistedAuthenticator returns an Authenticator that skips the underlying
+// auther when the client's IP address matches one of the given patterns.
+// Patterns may be IP addresses (e.g. "192.168.1.1") or CIDR ranges
+// (e.g. "10.0.0.0/8"). Invalid patterns are silently ignored.
+func WhitelistedAuthenticator(auther auth.Authenticator, patterns []string) auth.Authenticator {
+	var ips []net.IP
+	var inets []*net.IPNet
+	for _, pattern := range patterns {
+		pattern = strings.TrimSpace(pattern)
+		if pattern == "" {
+			continue
+		}
+		if ip := net.ParseIP(pattern); ip != nil {
+			ips = append(ips, ip)
+			continue
+		}
+		if _, inet, err := net.ParseCIDR(pattern); err == nil {
+			inets = append(inets, inet)
+		}
+	}
+	return &whitelistedAuthenticator{
+		auther:      auther,
+		ipMatcher:   matcher.IPMatcher(ips),
+		cidrMatcher: matcher.CIDRMatcher(inets),
+	}
+}
+
+func (w *whitelistedAuthenticator) Authenticate(ctx context.Context, user, password string, opts ...auth.Option) (string, bool) {
+	if srcAddr := xctx.SrcAddrFromContext(ctx); srcAddr != nil {
+		host, _, err := net.SplitHostPort(srcAddr.String())
+		if err == nil && host != "" {
+			if w.ipMatcher.Match(host) || w.cidrMatcher.Match(host) {
+				return "", true
+			}
+		}
+	}
+	if w.auther != nil {
+		return w.auther.Authenticate(ctx, user, password, opts...)
+	}
+	return "", false
+}