Add ability to load (deserialize) objects from other namespaces (#7726)

I have some Prompt subclasses in my project that I'd like to be able to
deserialize in callbacks. Right now `loads()`/`load()` will bail when it
encounters my object, but I know I can trust the objects because they're
in my own projects.

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libs/langchain/langchain/load/load.py

@@ -1,7 +1,7 @@
 import importlib
 import json
 import os
-from typing import Any, Dict, Optional
+from typing import Any, Dict, List, Optional
 
 from langchain.load.serializable import Serializable
 
@@ -9,8 +9,16 @@ from langchain.load.serializable import Serializable
 class Reviver:
     """Reviver for JSON objects."""
 
-    def __init__(self, secrets_map: Optional[Dict[str, str]] = None) -> None:
+    def __init__(
+        self,
+        secrets_map: Optional[Dict[str, str]] = None,
+        valid_namespaces: Optional[List[str]] = None,
+    ) -> None:
         self.secrets_map = secrets_map or dict()
+        # By default only support langchain, but user can pass in additional namespaces
+        self.valid_namespaces = (
+            ["langchain", *valid_namespaces] if valid_namespaces else ["langchain"]
+        )
 
     def __call__(self, value: Dict[str, Any]) -> Any:
         if (
@@ -43,8 +51,7 @@ class Reviver:
         ):
             [*namespace, name] = value["id"]
 
-            # Currently, we only support langchain imports.
+            if namespace[0] not in self.valid_namespaces:
-            if namespace[0] != "langchain":
                 raise ValueError(f"Invalid namespace: {value}")
 
             # The root namespace "langchain" is not a valid identifier.
@@ -66,14 +73,21 @@ class Reviver:
         return value
 
 
-def loads(text: str, *, secrets_map: Optional[Dict[str, str]] = None) -> Any:
+def loads(
+    text: str,
+    *,
+    secrets_map: Optional[Dict[str, str]] = None,
+    valid_namespaces: Optional[List[str]] = None,
+) -> Any:
     """Load a JSON object from a string.
 
     Args:
         text: The string to load.
         secrets_map: A map of secrets to load.
+        valid_namespaces: A list of additional namespaces (modules)
+            to allow to be deserialized.
 
     Returns:
 
     """
-    return json.loads(text, object_hook=Reviver(secrets_map))
+    return json.loads(text, object_hook=Reviver(secrets_map, valid_namespaces))