[keyserver][lib][native] Add urlPrefix to keyserverStore

Summary:
issue: https://linear.app/comm/issue/ENG-4459/refactor-urlprefix-field
urlPrefix needs to be moved to keyserverStore. In next diffs I will start using this value and remove the urlPrefix from where it was previously (BaseAppState).
For now we are updating the urlPrefix of "Ashoat's keyserver" when we want to connect to a different keyserver. During https://linear.app/comm/issue/ENG-4130/settings-panel-for-keyserver-selection some logic will be written, that will allow to add a new keyserver instead. Then, setUrlPrefix will be used only if the keyserver changes it's url, not as a way of connecting to a different keyserver

Test Plan: Ran yarn flow-all

Reviewers: michal, kamil

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8813