Add new auditor rules for billing page

What does this MR do and why?

Resolves #351601 (closed)

This creates a new policy rule for auditors that allows them to view the billing page. The functionality is behind the auditor_billing_page_access feature flag.

Screenshots or screen recordings

	Before	After
Owner View - Group
Auditor View - Group		(same as owner)
Guest View - Group
All users view - Profile		No change

How to set up and validate locally

• Setup and run CustomersDot
• Within your gitlab terminal window, run export GITLAB_SIMULATE_SAAS=1
• If GDK is already running, restart it
• Run rails c to open the rails console
• Run ::Gitlab::CurrentSettings.update(check_namespace_plan: true) and ::Gitlab::CurrentSettings.update(automatic_purchased_storage_allocation: true)
• Enable auditor_billing_page_access feature flag

Group Billing Page

• Ensure you have a group where you're an owner, or create one
• Verify that you can still access groups/<group-name>/-/billings
• Create an auditor user, invite them to the group, then impersonate as that user
• Verify that you can still access groups/<group-name>/-/billings
  • The sidebar button for the billings page will not be present. This will be added in a follow up MR.
• Attempt to access groups/<group-name>/-/billings with a guest user that's a group member but not an auditor. This should still 404.

Profile Billing Page

• Regression test that all user types are able to access :3000/-/profile/billings

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.