Refactor LoadBalancing::LoadBalancer to support scoping it to different models

What does this MR do?

This changes Gitlab::Database::LoadBalancing::LoadBalancer so it no longer explicitly depends on ActiveRecord::Base, and instead allows you to change the scope using an argument passed to initialize. This is one of the building blocks needed to add support for multiple databases to our load balancer.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
  • This is a backstage-only change, so no changelog is needed
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.