Reflect Istio option in API and limit auth options for it

We don't have OIDC in the adapter today. Therefore, we don't want customers to be able to enable it via UI or API when Istio is selected as integration option.

However, as pointed out by phala@redhat.com in this comment the API still allows people to set OIDC when using Istio and it shouldn't be available for Istio.

To Reproduce

1) Navigate to API docs in UI.
2) Select Service Create endpoint
3) Use

backend_version: oidc

and

deployment_option: service_mesh_istio

4) Send request
OR
1) Use curl command like this:

curl -v  -X POST "<ADMIN_URL>/admin/api/services.xml" -d 'access_token=<ACCESS_TOKEN>&name=new-api&deployment_option=service_mesh_istio&backend_version=oidc'