cmd/clef: encrypt the master seed on disk (#17704)

* cmd/clef: encrypt master seed of clef

Signed-off-by: YaoZengzeng <yaozengzeng@zju.edu.cn>

* keystore: refactor for external use of encryption

* clef: utilize keystore encryption, check flags correctly

* clef: validate master password

* clef: add json wrapping around encrypted master seed

signer/core/api.go

@@ -197,6 +197,12 @@ type (
 	Message struct {
 		Text string `json:"text"`
 	}
+	PasswordRequest struct {
+		Prompt string `json:"prompt"`
+	}
+	PasswordResponse struct {
+		Password string `json:"password"`
+	}
 	StartupInfo struct {
 		Info map[string]interface{} `json:"info"`
 	}