Make ser/de configurable in checkpointer classes

- Remove pydantic usage from base checkpointer class
- Make serialization configurable for all existing checkpointer classes, you can eg use dill or json instead of pickle

tests/memory_assert.py

@@ -1,18 +1,36 @@
 from collections import defaultdict
+from typing import Any
 
-from langchain_core.pydantic_v1 import Field
-
-from langgraph.checkpoint.base import Checkpoint, CheckpointAt, copy_checkpoint
+from langgraph.checkpoint.base import (
+    Checkpoint,
+    CheckpointAt,
+    SerializerProtocol,
+    copy_checkpoint,
+)
 from langgraph.checkpoint.memory import MemorySaver
 
 
+class NoopSerializer(SerializerProtocol):
+    def loads(self, data: bytes) -> Any:
+        return data
+
+    def dumps(self, obj: Any) -> bytes:
+        return obj
+
+
 class MemorySaverAssertImmutable(MemorySaver):
-    storage_for_copies: defaultdict[str, dict[str, Checkpoint]] = Field(
-        default_factory=lambda: defaultdict(dict)
-    )
+    serde = NoopSerializer()
 
     at = CheckpointAt.END_OF_STEP
 
+    storage_for_copies: defaultdict[str, dict[str, Checkpoint]]
+
+    def __init__(
+        self, *, serde: SerializerProtocol | None = None, at: CheckpointAt | None = None
+    ) -> None:
+        super().__init__(serde=serde, at=at)
+        self.storage_for_copies = defaultdict(dict)
+
     def put(self, config: dict, checkpoint: Checkpoint) -> None:
         # assert checkpoint hasn't been modified since last written
         thread_id = config["configurable"]["thread_id"]