Add ability to choose "Vendored" as an option from the models list dropdown for a AI features

After the backend work and Duo Health check work has been complete for vendored models, as last step, we should add the ability for admins to choose Vendored (or maybe GitLab AI Vendor model) as the option from the list of models dropdown on the admin/gitlab_duo/self_hosted page.

Let us also add a

::License.current&.online_cloud_license?

check before showing this option in the dropdown. This is because only instances with cloud-licenses can connect to cloud-connected AIGW. So, if they are on an offline license, there is no point in showing the "Vendored" option.