Feature flag for disabling free access for custom models

What does this MR do and why?

This MR introduces a feature flag that allows disabling free access to self-hosted features. We plan to introduce licensing in %17.4 and it would be great to have a way to test in until the cut off date (17 Oct 2024).

If a customer received the trial license before 17 Oct and is ready to use it instead of relying on the free access, they can enable self_hosted_models_beta_ended feature flag that disables the free access and enables calculating the feature availability via the duo-enterprise database records. In this case, if anything goes wrong they can disable the feature flag and we can react to the feedback and deliver the fix before in %17.5 if necessary.

Related issue: https://gitlab.com/gitlab-org/gitlab/-/issues/485491+