[lib] Extract shared specs to a lib

Summary:
A part of a spec will be used in a lib. We can't migrate the whole spec there because fetch function is server-specific.

Depends on D9008

https://linear.app/comm/issue/ENG-4813/migrate-getclientresponsesselector-to-a-spec

Test Plan: Flow

Reviewers: kamil, bartek, inka

Reviewed By: bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D9009