[Feature][main]reconstruction kvpool connector to ascend connector (#4438)

### What this PR does / why we need it?
1.In short, we renamed the existing MooncakeStoreConnector to
AscendStoreConnector and extracted the storage engine interaction logic
into a new Backend class.
Associated RFC：https://github.com/vllm-project/vllm-ascend/issues/4329
2.Fixed the issue where the number of input parameters for the connector
was incorrect, introduced in vllm 0.11.2
### Does this PR introduce _any_ user-facing change?
change MooncakeStoreConnector to AscendStoreConnector
### How was this patch tested?

- vLLM version: v0.11.2

---------

Signed-off-by: fems14 <1804143737@qq.com>

vllm_ascend/distributed/kvpool/backend/backend.py

@@ -0,0 +1,29 @@
+from abc import ABC, abstractmethod
+
+from vllm.config import ParallelConfig
+
+
+class Backend(ABC):
+
+    def __init__(self, parallel_config: ParallelConfig):
+        pass
+
+    def set_device(self):
+        pass
+
+    def register_buffer(self, ptrs: list[int], lengths: list[int]):
+        pass
+
+    @abstractmethod
+    def exists(self, keys: list[str]) -> list[int]:
+        pass
+
+    @abstractmethod
+    def put(self, keys: list[str], addrs: list[list[int]],
+            sizes: list[list[int]]):
+        pass
+
+    @abstractmethod
+    def get(self, keys: list[str], addrs: list[list[int]],
+            sizes: list[list[int]]):
+        pass