Add disabled_direct_code_suggestions setting

What does this MR do and why?

Adds disabled_direct_code_suggestions application setting which can be used to disable direct connections to AI Gateway for code completions.

If this admin sets this option, then direct_access endpoint (used by clients to obtain connection details) will return an error code so client will fall-back to use indirect connections through Rails.

Related to #462791 (closed)

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Before	After

How to set up and validate locally

1. Add a valid codesuggestion addon (https://docs.gitlab.com/ee/development/code_suggestions/#setup-instructions-to-use-gdk-with-the-code-suggestions-add-on)
2. Navigate to Admin->Settings and check that there is an option to disable direct connections