This diff adds keyserver url validation logic to the `onPressSave` callback in the add keyserver screen. We also now display an error message if for any reason the keyserver url inputed by the user is not valid
Linear task: https://linear.app/comm/issue/ENG-4917/update-onpress-of-goadd-button-in-customservermodal-to-add-keyserver
Depends on D9666