cmd/clef, signer: refresh tutorial, fix noticed issues (#19774)

* cmd/clef, signer: refresh tutorial, fix noticed issues

* cmd/clef, signer: support removing stored keys (delpw + rules)

* cmd/clef: polishes + Geth integration in the tutorial

signer/storage/aes_gcm_storage.go

@@ -53,7 +53,7 @@ func NewAESEncryptedStorage(filename string, key []byte) *AESEncryptedStorage {
 	}
 }
 
-// Put stores a value by key. 0-length keys results in no-op
+// Put stores a value by key. 0-length keys results in noop.
 func (s *AESEncryptedStorage) Put(key, value string) {
 	if len(key) == 0 {
 		return
@@ -75,27 +75,41 @@ func (s *AESEncryptedStorage) Put(key, value string) {
 	}
 }
 
-// Get returns the previously stored value, or the empty string if it does not exist or key is of 0-length
-func (s *AESEncryptedStorage) Get(key string) string {
+// Get returns the previously stored value, or an error if it does not exist or
+// key is of 0-length.
+func (s *AESEncryptedStorage) Get(key string) (string, error) {
 	if len(key) == 0 {
-		return ""
+		return "", ErrZeroKey
 	}
 	data, err := s.readEncryptedStorage()
 	if err != nil {
 		log.Warn("Failed to read encrypted storage", "err", err, "file", s.filename)
-		return ""
+		return "", err
 	}
 	encrypted, exist := data[key]
 	if !exist {
 		log.Warn("Key does not exist", "key", key)
-		return ""
+		return "", ErrNotFound
 	}
 	entry, err := decrypt(s.key, encrypted.Iv, encrypted.CipherText, []byte(key))
 	if err != nil {
 		log.Warn("Failed to decrypt key", "key", key)
-		return ""
+		return "", err
+	}
+	return string(entry), nil
+}
+
+// Del removes a key-value pair. If the key doesn't exist, the method is a noop.
+func (s *AESEncryptedStorage) Del(key string) {
+	data, err := s.readEncryptedStorage()
+	if err != nil {
+		log.Warn("Failed to read encrypted storage", "err", err, "file", s.filename)
+		return
+	}
+	delete(data, key)
+	if err = s.writeEncryptedStorage(data); err != nil {
+		log.Warn("Failed to write entry", "err", err)
 	}
-	return string(entry)
 }
 
 // readEncryptedStorage reads the file with encrypted creds