feat(grpc): add support for fetching empty classrooms

Implement the `TASK_TYPE_GET_EMPTY_CLASSROOM` task type to allow users to fetch available classroom information.

- Add `TASK_TYPE_GET_EMPTY_CLASSROOM` to the `TaskType` enum in protobuf.
- Define `GetEmptyClassroomPayload` and `EmptyClassroomResultData` messages.
- Implement `getEmptyClassrooms`

grpc/client.go

@@ -77,10 +77,11 @@ func (c *Client) register() error {
 				RunnerId: c.runnerID,
 				Version:  c.version,
 				Capabilities: map[string]string{
-					"TASK_TYPE_GET_SCHEDULE": "true",
-					"TASK_TYPE_GET_GRADES":   "true",
-					"TASK_TYPE_GET_EXAMS":    "true",
-					"TASK_TYPE_LOGIN":        "true",
+					"TASK_TYPE_GET_SCHEDULE":        "true",
+					"TASK_TYPE_GET_GRADES":          "true",
+					"TASK_TYPE_GET_EXAMS":           "true",
+					"TASK_TYPE_LOGIN":                "true",
+					"TASK_TYPE_GET_EMPTY_CLASSROOM":  "true",
 				},
 			},
 		},