Allows cascading package forward settings from admin to group

What does this MR do and why?

Adds functionality to cascade package forwarding settings from admin to group

This MR leverages GraphQL changes implemented in Add cascading package settings to GraphQL (!100526 - merged) and backend functionality implemented in Cascading namespace settings for package forwar... (#360267 - closed)

Even though lines changed is larger than usual, most of those changes are just tests.

Screenshots or screen recordings

Admin application settings

Group settings

When disabled

How to set up and validate locally

1. Visit /admin/application_settings/ci_cd
2. Make sure new settings are visible & can be updated.
3. Enable maven_central_request_forwarding & make sure maven settings are also visible.
4. Visit group Settings > Packages & registry settings.
5. Make sure new section for settings is visible. Enable maven_central_request_forwarding, make sure maven settings are also visible.
6. Enabling Enforce PyPI setting for all subgroups at parent level should disable settings at the child level.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #360487 (closed)