Add an option/setting to control if direct connections can be used

Some customers use proxies for outgoing network traffic and using direct connections may not always be possible for some self-managed deployments. For that reason, there should be a way for self-managed customers to disable direct connections if needed.

This could be done by adding a long-term ops feature flag which would be checked in api/v4/code_suggestions/direct_access endpoint (similarly how we check temporary code_suggestions_direct_access feature flag in this endpoint).

Update: because ops feature flag is not supported on dedidcated servers (though there is a low chance customers would need to disable it there), a more universal option would be add an application setting.

Related to &13252 (comment 1859083558)