[lib] Introduce UserIdentityCache

Summary:
This is modelled after ENSCache and FCCache, but it's implemented as a React component since it needs access to a React context.

This addresses part of ENG-9002.

Depends on D13117

Test Plan: I deployed a release build to my iOS device with some logging inside both IdentityServiceContentProvider and UserInfosHandler. I confirmed that the latter only ever queried for users once, even though the latter queried multiple times

Reviewers: varun, will

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13126