Users parameter for cross-validate

[blondered]

Feature Description

users parameter in cross-validate will accept the list of external user ids to run all experiments only on these users.

Why this feature?

This will help to solve a common task to calculate metrics only on specific subset of users

Additional context

No response

[nishantguvvada]

Hi @blondered, I hope it's okay for me to go over it. I've created a pull request in response to the issue: #112