[lib] Add $ReadOnly wrapper around type spreads in socket-types.js

Summary:
Noticed these were missing. For whatever reason in Flow, if you spread something it loses the read-only-ness, and it needs to be re-applied.

Depends on D12366

Test Plan: Flow

Reviewers: tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D12367