feat: add signtool to plugin repository

- Migrate signing tool from GoTunnel main project
- Self-contained, no external dependencies
- Updated CI workflow to build locally

tools/signtool/sign/payload.go

@@ -0,0 +1,70 @@
+package sign
+
+import (
+	"crypto/ed25519"
+	"crypto/sha256"
+	"encoding/hex"
+	"encoding/json"
+	"fmt"
+	"strings"
+	"time"
+)
+
+// PluginPayload 插件签名载荷
+type PluginPayload struct {
+	Name       string `json:"name"`
+	Version    string `json:"version"`
+	SourceHash string `json:"source_hash"`
+	KeyID      string `json:"key_id"`
+	Timestamp  int64  `json:"timestamp"`
+}
+
+// SignedPlugin 已签名的插件
+type SignedPlugin struct {
+	Payload   PluginPayload `json:"payload"`
+	Signature string        `json:"signature"`
+}
+
+// NormalizeSource 规范化源码
+func NormalizeSource(source string) string {
+	normalized := strings.ReplaceAll(source, "\r\n", "\n")
+	normalized = strings.ReplaceAll(normalized, "\r", "\n")
+	return strings.TrimRight(normalized, " \t\n")
+}
+
+// HashSource 计算源码哈希
+func HashSource(source string) string {
+	normalized := NormalizeSource(source)
+	hash := sha256.Sum256([]byte(normalized))
+	return hex.EncodeToString(hash[:])
+}
+
+// CreatePayload 创建签名载荷
+func CreatePayload(name, version, source, keyID string) *PluginPayload {
+	return &PluginPayload{
+		Name:       name,
+		Version:    version,
+		SourceHash: HashSource(source),
+		KeyID:      keyID,
+		Timestamp:  time.Now().Unix(),
+	}
+}
+
+// SignPlugin 签名插件
+func SignPlugin(priv ed25519.PrivateKey, payload *PluginPayload) (*SignedPlugin, error) {
+	data, err := json.Marshal(payload)
+	if err != nil {
+		return nil, fmt.Errorf("marshal payload: %w", err)
+	}
+	sig := SignBase64(priv, data)
+	return &SignedPlugin{Payload: *payload, Signature: sig}, nil
+}
+
+// EncodeSignedPlugin 编码为 JSON
+func EncodeSignedPlugin(sp *SignedPlugin) (string, error) {
+	data, err := json.Marshal(sp)
+	if err != nil {
+		return "", err
+	}
+	return string(data), nil
+}