Add DB field for an instance-level Beta AI toggle (Duo Chat)

What does this MR do and why?

First part https://gitlab.com/gitlab-org/gitlab/-/issues/425047 (not including UI)

Adds a new instance-wide toggle (DB value) to enable/disable Beta AI features.
Initially, it will be used to enable/disable Duo Chat.

We don't want UI changes to appear %16.7, so the change is only scoped to DB value.
groupai framework can use it while working on the Duo Chat client (to add permissions, for example: #431563 (comment 1664655122)).

How to set up and validate locally

Switch to this branch

Run migrations: bin/rails db:migrate

In the Rails console, check that accessing the field does not throw an error:

[3] pry(main)> ::Gitlab::CurrentSettings.instance_level_ai_beta_features_enabled
=> false

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.

Related to #425047