[lib] add relative user info to users in messageReactionInfo

Summary:
Right now users in messageReactionInfo returns a set of user ids of all the users who have liked a message. Now that we want to show information about the user like their username, I needed to convert users from a set of user IDs to an array of RelativeUserInfos. Also needed to make some changes to the reaction-utils test suite since we are no longer using a set as our data structure.

Linear Task: ENG-2718

Test Plan:
Logged out messageReactionInfo to show that my changes are behaving as expected (see screenshots below):

Before:

After:

Reviewers: tomek, atul

Reviewed By: tomek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6301