[UDP] Pull does not update Callback URL field

After installing and configuring the UDP package, the user can configure a callback URL via the UDP Settings inspector. Currently using the "Pull" feature will not update this field even if a different value is in its place than on the server.

Steps to reproduce:
- In Unity 2019.3, create a new project and install the Universal Distribution Portal (UDP) package (alternatively download the attached project)
- Menu: Window > Universal Distribution Portal > Settings
-Inspector (UDP Settings) > Register the project with Unity Cloud Services if the UDP Settings Inspector pane indicates it is not already
- Inspector (UDP Settings) > Settings > Callback URL > Set to any URL
- Push the update to the UDP server
- Change the callback URL without pushing, - Inspector (UDP Settings) > Settings > Callback URL > Set to any URL (different than before)
- Instead of Pushing the changes, Pull
- Observe the configured Callback URL which was pushed to the server does not replace the Callback URL field
- (Optionally) Restart the editor and observe the Callback URL is now the correct URL stored on the UDP server