Timesheet API: Refactor getUsers and add admin functionality

[izzyconner]

Currently, the getUsers endpoint works by querying cognito separately for each user. This has some drawbacks, especially the 30 call / second limit for the read requests.

We need to fully build out an alternative way to get all users of a certain role from Cognito, and then filter by userId on our end. This is also going to be necessary for admin functionality.

Admins should, by default, receive all companies and associated user data unless otherwise specified via companyId query. Currently, the user Controller

Finally, once these changes are in place, we should also move most of the logic into UserService out of UserController

[izzyconner]

started some work, not nearly done but will push for now.