[Misc] Introduce ec_both role EC (encoder cache) connector (#34182)

Signed-off-by: Qi Wang <qiwa@nvidia.com>

vllm/distributed/ec_transfer/ec_connector/base.py

@@ -63,6 +63,7 @@ class ECConnectorBase(ABC):
         self._role = role
         if vllm_config.ec_transfer_config is not None:
             self._is_producer = vllm_config.ec_transfer_config.is_ec_producer
+            self._is_consumer = vllm_config.ec_transfer_config.is_ec_consumer
         else:
             raise ValueError("ec_transfer_config must be set for ECConnectorBase")
 
@@ -74,6 +75,10 @@ class ECConnectorBase(ABC):
     def is_producer(self) -> bool:
         return self._is_producer
 
+    @property
+    def is_consumer(self) -> bool:
+        return self._is_consumer
+
     # ==============================
     # Worker-side methods
     # ==============================