Extract members_with_parents into another class

What does this MR do and why?

NOTE: It will be easier to review commit-by-commit

Related issue: #422405 (closed), #417455 (closed), https://gitlab.com/gitlab-org/gitlab/-/issues/421527

Extract members_with_parents into another class

Reduces the size of the Group class. We move code that is not strictly related to a group to its own class - this code is more to do with the hierarchy of groups' members.

Why? See https://docs.gitlab.com/ee/development/software_design.html#taming-omniscient-classes

Also it will make Cells: Fix cross-joins arising from members (#422405 - closed) easier to fix

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Method	Before	After	Notes and formatted queries
EE #member?	https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/23618/commands/75906 https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/23618/commands/75907	https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/23618/commands/75900	!135328 (comment 1632614420)
members_with_parents(only_active_users: true)	https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/23618/commands/75903	https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/23618/commands/75901	!135328 (comment 1632614494)
members_with_parents(only_active_users: false)	https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/23618/commands/75904	https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/23618/commands/75902	!135328 (comment 1632614494)
authorizable_members_with_parents	https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/23618/commands/75923	https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/23618/commands/75922	!135328 (comment 1632614537)

• before.sql
• after.sql

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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