Added support for mongoDB KV store (#543)

Added the support for mongoDB as KV store
validated in mongodb, it is able to store agent data, session data and
turn data
<img width="1332" alt="image"
src="https://github.com/user-attachments/assets/867700a4-b9ee-4a3c-8278-f39074d39d56">
this is how run.yaml would look:
```
    config:
      persistence_store:
        type: mongodb
        namespace: null
        host: localhost
        port: 27017
        db: llamastack
        user: ""
        password: ""
        collection_name: llamastack_kvstore
```

---------

Co-authored-by: shrinitgoyal <shrinit.goyal@engati.com>

llama_stack/providers/utils/kvstore/mongodb/mongodb.py

@@ -0,0 +1,69 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+import logging
+from datetime import datetime
+from typing import List, Optional
+
+from pymongo import MongoClient
+
+from llama_stack.providers.utils.kvstore import KVStore, MongoDBKVStoreConfig
+
+log = logging.getLogger(__name__)
+
+
+class MongoDBKVStoreImpl(KVStore):
+    def __init__(self, config: MongoDBKVStoreConfig):
+        self.config = config
+        self.conn = None
+        self.collection = None
+
+    async def initialize(self) -> None:
+        try:
+            conn_creds = {
+                "host": self.config.host,
+                "port": self.config.port,
+                "username": self.config.user,
+                "password": self.config.password,
+            }
+            conn_creds = {k: v for k, v in conn_creds.items() if v is not None}
+            self.conn = MongoClient(**conn_creds)
+            self.collection = self.conn[self.config.db][self.config.collection_name]
+        except Exception as e:
+            log.exception("Could not connect to MongoDB database server")
+            raise RuntimeError("Could not connect to MongoDB database server") from e
+
+    def _namespaced_key(self, key: str) -> str:
+        if not self.config.namespace:
+            return key
+        return f"{self.config.namespace}:{key}"
+
+    async def set(
+        self, key: str, value: str, expiration: Optional[datetime] = None
+    ) -> None:
+
+        key = self._namespaced_key(key)
+        update_query = {"$set": {"value": value, "expiration": expiration}}
+        self.collection.update_one({"key": key}, update_query, upsert=True)
+
+    async def get(self, key: str) -> Optional[str]:
+        key = self._namespaced_key(key)
+        query = {"key": key}
+        result = self.collection.find_one(query, {"value": 1, "_id": 0})
+        return result["value"] if result else None
+
+    async def delete(self, key: str) -> None:
+        key = self._namespaced_key(key)
+        self.collection.delete_one({"key": key})
+
+    async def range(self, start_key: str, end_key: str) -> List[str]:
+        start_key = self._namespaced_key(start_key)
+        end_key = self._namespaced_key(end_key)
+        query = {
+            "key": {"$gte": start_key, "$lt": end_key},
+        }
+        cursor = self.collection.find(query, {"value": 1, "_id": 0}).sort("key", 1)
+        return [doc["value"] for doc in cursor]