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`

parser/encoding.go

@@ -2,6 +2,7 @@ package parser
 
 import (
 	"bytes"
+	"encoding/json"
 	"io"
 	"net/http"
 	"strings"
@@ -71,4 +72,12 @@ func isValidUTF8(data []byte) bool {
 		}
 	}
 	return true
+}
+
+func decodeJSONResponse(resp *http.Response, v any) error {
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return err
+	}
+	return json.NewDecoder(bytes.NewReader(body)).Decode(v)
 }