* Allow auth plugins to redirect to external url
Add a new optional field to `RegisterServerExternalAuthenticatedResult`,
the object passed to the `userAuthenticated` callback used by auth plugins.
The server code uses this to redirect to an external website if it is set.
Left TODO:
- This code has been tested manually but a test case is still missing.
- Here or in the plugin, the redirect urls must be limited to values configurable by admins.
* rename to URI for consistency
* add test for the new parameter
* address review comments
- correct syntax for optional parameter
- handle the case where `externalAuthToken` has query parameters included
