[native] introduce HeaderRightTextButton

Summary: In the keyserver selection screen we want to add an add button to the right header. I noticed that the Linked devices screen also has the same thing, so as a preliminary step to introducing the add button to the keyserver selection screen, I factored out the shared logic of this button so that it can be used in both places

Test Plan: flow and confirmed that the add button in the linked devices screen still works as expected

Reviewers: rohan, inka, michal

Reviewed By: rohan

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9586