fix: Add missing import json to depth_estimation_node.py

Resolves NameError: name 'json' is not defined (#18).
The json module is used in _serialize_camera_data() and error handling
but was never imported when DA3 camera pose estimation was added in v1.3.4.

Bump version to 1.3.7.

depth_estimation_node.py

@@ -1,4 +1,5 @@
 import os
+import json
 import numpy as np
 import torch
 import traceback