Add additional override flag to handle opting out

What does this MR do and why?

Add additional override feature flag. flag to handle opting out.

To selectively disable by actor, we need to add a second flag to allow this as defined in our documentation. https://docs.gitlab.com/ee/development/feature_flags/controls.html#selectively-disable-by-actor

Screenshots or screen recordings

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

Flag disabled (default behaviour)	After (internal projects, grandmothered in)

How to set up and validate locally

• Create a namespace with an ultimate licence.
• Ensure experiment and beta features are enabled at the group level.
• Create a new project within that namespace
• Go to Analytics -> Analytics Dashboards
• Enable the product_analytics_billing flag for all projects.
• Compare the before and after when enabling product_analytics_billing_override for just that namespace. /assign me