Standardize service constructors

What does this MR do and why?

Closes: Standardize Service Constructors (#408770 - closed)

• Standardizes service layer constructors based on https://docs.gitlab.com/ee/development/reusing_abstractions.html#service-classes in terms of current_user
• Adds comments explaining why we are intentionally deviating from the standards in some ways
• Adds two TODOs to add auth to service layer.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.