[lib] modify reaction message generatesNotifs function to return a pushType based on the reaction action

Summary:
Modified generatesNotifs in reaction-message-spec to return notif if the reaction action is equal to add_reaction and rescind if the reaction action is equal to remove_reaction

Won't land this diff until the entire stack for this task is accepted and ready to land. The current changes would cause notifs to no longer send if a user were to unlike a message

Linear Task: ENG-2644

Test Plan: This will be further tested in subsequent diffs

Reviewers: atul, tomek

Reviewed By: tomek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6307