[lib] Move update type enum into a separate file

Summary:
Moved the enum to fix dependency cycles (that would be introduced in the later diffs).

Depends on D7785

Test Plan: Run yarn jest-all and yarn flow-all

Reviewers: kamil, tomek, ashoat

Reviewed By: ashoat

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7788