Create /config/data/default API endpoint to serve default khoj config

This can ease configuring khoj from the different interfaces

- Don't need to know all the (default) config used by khoj.
- Just get default config by calling the above API endpoint.
- Then modify desired portions and call POST /api/config/data to
  configure khoj.

src/routers/api.py

@@ -21,6 +21,10 @@ logger = logging.getLogger(__name__)
 
 
 # Create Routes
+@api.get('/config/data/default')
+def get_default_config_data():
+    return constants.default_config
+
 @api.get('/config/data', response_model=FullConfig)
 def get_config_data():
     return state.config