[lib] Add containedThreadInfos selector

Summary:
Part of ENG-4319.

A new selector is needed to obtain contained chats. There exists childThreadInfos selector, but it operates on parentThreadID rather than on containingThreadID.

Depends on D8686.

Test Plan: Tested later in the stack.

Reviewers: bartek, tomek, inka, michal, ashoat

Reviewed By: bartek, inka, ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8526