chore: adjust message/command structure and write unit tests

ref: N25B-205

src/control_backend/schemas/ri_message.py

@@ -0,0 +1,20 @@
+from enum import Enum
+from typing import Any, Literal
+
+from pydantic import BaseModel, Field, ValidationError
+
+
+class RIEndpoint(str, Enum):
+    SPEECH = "actuate/speech"
+    PING = "ping"
+    NEGOTIATE_PORTS = "negotiate/ports"
+
+
+class RIMessage(BaseModel):
+    endpoint: RIEndpoint
+    data: Any   
+
+
+class SpeechCommand(RIMessage):
+    endpoint: RIEndpoint = RIEndpoint(RIEndpoint.SPEECH)
+    data: str