[lib/native] add keyserver url validation logic and error handling to the add keyserver screen

Summary:
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

Test Plan:
Please see the demo video below

Reviewers: inka, rohan, michal

Reviewed By: inka

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9667