Increse billable users when using custom roles

What does this MR do and why?

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Graphql

• Run GitLab as SaaS
• Assign a group Ultimate or gold plan in admin area (eg. https://gdk.test:3443/admin/groups/flightjs/edit)
• Try the following graphql query with and without memberRoleId param (https://gdk.test:3443/-/graphql-explorer)
• The newBillableUserCount should be higher with the memberRoleId param

query {
  group(fullPath: "flightjs") {
    id
    gitlabSubscriptionsPreviewBillableUserChange(
      addUserIds: USER_IDs
      role: GUEST
      memberRoleId: ROLE_ID
    ) {
      willIncreaseOverage
      seatsInSubscription
      newBillableUserCount
    }
  }
}

For the UI:

• this MR does not change UI, guidance for future reference

1. Enable the following feature flags:
   • echo "Feature.enable(:overage_members_modal)" | gdk rails console
   • echo "Feature.enable(:show_overage_on_role_promotion)" | gdk rails console
   • echo 'Feature.enable(:custom_roles_in_members_page)' | gdk rails console
2. In the member page, assign a custom role to the member. A modal with a warning should be displayed.
3. In the member page, invite a member and assign him/her a custom role. A modal with a warning should be displayed.

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 #428043 (closed)