[lib] Move WebInitialReduxStateResponse to lib

Summary:
Move WebInitialReduxStateResponse (and WebInitialKeyserverInfo) to lib.

We will then:

1. Split into Client and Server variants
2. Make the client one take ThreadStore and server can take LegacyThreadStore

Depends on D10894

Test Plan: CI + flow + close reading of changes. Used IDE Refactor -> Move tool to make this change.

Reviewers: ashoat, ginsu, tomek

Reviewed By: ginsu

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10895