Allow AI features to be available on Self-Managed

What does this MR do and why?

Allow enabling AI features on SM instances.

Changes include: small class that is responsible for checking feature flags based on new class that is SSOT for all ai features.

MR acceptance checklist

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Before	After

How to set up and validate locally

1. Run GDK in NON-Saas mode.
2. Verify on master that you cannot see standalone ai features (for example: generating issue description).
3. In rails console disable the feature flag that is used on SaaS

   Feature.disable(:ai_global_switch)

4. In this file, enable one feature for self-managed, by changing self-managed value to true. I checked it work without any additional changes for generate_description.
5. Two changes that are not connected to this MR but are required or helpful:

• in https://gitlab.com/gitlab-org/gitlab/-/blob/18049920a4336431d2dc35580b2550f63eaa6017/ee/app/workers/llm/completion_worker.rb#L33-33 - change perform_async to perform_inline
• in this method comment out full if-clause.

1. Visit page for generating new issue and check if there is a button that indicates this feature is active.
2. Click on the button and generate description.