Add feature toggle for security policy custom CI YAML

We want to release Pipeline Execution Action (Custom CI YAML Suppo... (&7312 - closed) as an experiment in %16.7. The feature is behind the compliance_pipeline_in_policies feature flag but an experiment requires a more convenient way to opt in.

Offer a way to opt-in with minimal friction. For example, needing to flip a feature flag is too much friction, but a group or project-level setting in the UI is not.

The toggle should be in form of a group level setting nested under Settings => CI/CD. We might be able to use the cascading setting framework to implement it.