Use combined pagination in enable scanners wizard

What does this MR do and why?

Use combined pagination in enable scanners wizard

List only direct descendants in select projects step of the wizard
with subgroups first, then projects, in pages of 20 items
by reusing the sequential pagination from security inventory
Call profile application mutation with groupIds and projectIds
Add feature flag that controls which queries/pagination this uses

References

• issue: [Frontend] Enable scanners wizard: Select proje... (#602374)
• feature flag rollout issue: [FF] `combined_pagination_in_scanner_wizard` --... (#605583)
• epic: [Security Contols] Scanner Enablement Wizard (gitlab-org#21299)
• related issue for introducing this combined pagination and using it in security inventory: [Frontend] Combine more than one type of item i... (#591670)

Screenshots or screen recordings

Before	After
list shows only projects	list includes subgroups
	subgroups are selectable items
all projects are shown in the list in pages of 20	all subgroups are shown first, then direct descendant projects, in combined pages of 20 items each
	subgroups are not shown when search/filters are applied
projects are shown in the review step, and the mutation is called with their ids as projectIds	selected subgroups are shown in the review step, and the mutation is called with selected project ids as projectIds and selected subgroup ids as groupIds

How to set up and validate locally

git checkout 602374-use-combined-pagination-in-wizard-select-projects-step
echo "Feature.enable(:combined_pagination_in_scanner_wizard)" | gdk rails c

1. go to a group that has at least one subgroup => Secure => Security configuration
2. click Enable scanners in the upper right corner to go to the wizard
3. click the card Advanced setup, then Start advanced setup to get to the selection step
4. subgroups that are in the group should show as selectable items in the list - select some
5. click Select scanners to continue, select at least one scanner, then click Review configuration
6. the subgroups you selected should appear in the Items card
7. click Apply configuration
8. the mutation should be called with the groupIds of the selected subgroups

MR acceptance checklist

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Related to #602374