[lib] Add chat mention SearchIndex selector

Summary:
Solution for ENG-4552.

This diff introduces utilities for storing and using calculated SearchIndex for each thread in threadStore.

Depends on D8899.

Test Plan: Tested later in the stack.

Reviewers: tomek, inka, rohan

Reviewed By: tomek, inka

Subscribers: atul, ashoat, tomek

Differential Revision: https://phab.comm.dev/D8900