finalize api for plugin events in backend

just need frontend impl now

backend/decky_loader/plugin/messages.py

@@ -0,0 +1,36 @@
+from typing import Any, TypedDict
+from enum import IntEnum
+from uuid import uuid4
+from asyncio import Event
+
+class SocketMessageType(IntEnum):
+    CALL = 0
+    RESPONSE = 1
+    EVENT = 2
+
+class SocketResponseDict(TypedDict):
+    type: SocketMessageType
+    id: str
+    success: bool
+    res: Any
+
+class MethodCallResponse:
+    def __init__(self, success: bool, result: Any) -> None:
+        self.success = success
+        self.result = result
+
+class MethodCallRequest:
+    def __init__(self) -> None:
+        self.id = str(uuid4())
+        self.event = Event()
+        self.response: MethodCallResponse
+    
+    def set_result(self, dc: SocketResponseDict):
+        self.response = MethodCallResponse(dc["success"], dc["res"])
+        self.event.set()
+    
+    async def wait_for_result(self):
+        await self.event.wait()
+        if not self.response.success:
+            raise Exception(self.response.result)
+        return self.response.result