Move trust score methods to User Trust Score class

What does this MR do and why?

New methods related to a user's trust score were added to the User class as it has an association with abuse_trust_scores. However, this is a poor practice as the class is currently overloaded and adding more methods to it adds complexities.

A better way of approaching this is by extracting these methods to a separate class - Abuse::UserTrustScore, a thin wrapper around the user object that is catered to returning data representing a user's trust score - !117853 (comment 1423070054).

Related issue - https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/406

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