Draft: Select limited user columns for gitlab git auth

What does this MR do?

Related to (but does not close) #325332 (closed)

Explores whether it's feasible to limit the columns selected for Gitlab::Auth class - used by Git and JWT.

Existing Query

SELECT "users".* 
FROM "users" 
WHERE (LOWER("users"."username") = LOWER('root')) 
LIMIT 1

Proposed Query

SELECT "users"."id", "users"."name", "users"."username", "users"."email", 
"users"."confirmed_at", "users"."locked_at", "users"."state", "users"."user_type",
 "users"."admin", "users"."auditor", "users"."encrypted_password", 
"users"."otp_required_for_login", "users"."last_activity_on", "users"."projects_limit", 
"users"."accepted_term_id", "users"."managed_group_id", "users"."password_expires_at", "users"."confirmation_sent_at" 
FROM "users" WHERE (LOWER("users"."username") = LOWER('root')) 
LIMIT 1

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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