[native] introduce temp onPressDisconnectKeyserver callback

Summary:
As of Nov 2023, disconnecting from a keyserver is outside the scope of the keyserver selection screen. However, we currently have the UI for this already implemented so as a temporary solution this diff has the onPressDisconnectKeyserver callback display an alert to the user, informing them that this feature is not quite ready yet

Depends on D9826

Test Plan:
Please see demo video below

Reviewers: inka, michal, rohan, ashoat

Reviewed By: rohan

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9828