[lib] introduce SyncedMetadata store spec

Summary:
This diff introduces the SyncedMetadata store ops spec. This will be used process operations on the integrity store

Depends on D11414

Test Plan: flow check. Functionality tested later in stack

Reviewers: kamil, ginsu, atul

Reviewed By: kamil, atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11415