[lib/native] lift staff provider into lib

Summary:
As I was building the build info page for web, I noticed that the hasStaffUserLoggedIn only lives in native. To get this field into web we should lift the staff context provider into lib so that it can be used on both platforms. This diff lifts the staff provider into lib so that it can be used in both web and native

Depends on D10825

Test Plan: flow + confirmed that there were no regressions with native

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10839