json: Implement json.Unmarshaler for messages.

This will allow using pgmockproxy output as ingestion data for pgmock.

sasl_initial_response.go

@@ -67,3 +67,26 @@ func (src SASLInitialResponse) MarshalJSON() ([]byte, error) {
 		Data:          hex.EncodeToString(src.Data),
 	})
 }
+
+// UnmarshalJSON implements encoding/json.Unmarshaler.
+func (dst *SASLInitialResponse) UnmarshalJSON(data []byte) error {
+	// Ignore null, like in the main JSON package.
+	if string(data) == "null" {
+		return nil
+	}
+
+	var msg struct {
+		AuthMechanism string
+		Data          string
+	}
+	if err := json.Unmarshal(data, &msg); err != nil {
+		return err
+	}
+	decodedData, err := hex.DecodeString(msg.Data)
+	if err != nil {
+		return err
+	}
+	dst.AuthMechanism = msg.AuthMechanism
+	dst.Data = decodedData
+	return nil
+}