Detect the presence of the istio proxy container in revision pod spec

[grantr]

/area API
/area networking
/kind feature

Expected Behavior

If the Istio sidecar proxy fails to be injected into a revision pod spec, networking to and from those pods will silently fail in subtle ways due to the absence of the sidecar. The user should know when this happens so it can be corrected.

Actual Behavior

If the Istio sidecar injector is missing or incorrectly configured, there is no indication that sidecar injection failed. For simple demo apps, there's no behavior change because normal Kubernetes Service routing still works. The failures will start when apps need to talk to other apps in the cluster or use more complex Istio routing rules.

Steps to Reproduce the Problem

1. When setting up an Elafros cluster, skip the sidecar injector configuration.
2. Create a revision.
3. Notice that the revision's deployment has no istio-proxy container.

Additional Info

The revision controller should look for the istio-proxy sidecar and if it's missing, set Ready=False with Reason=Missing istio proxy, or set a specific condition like NetworkProxyMissing (conforming to the condition recommendations in #351).

/cc @ian-mi

[bsnchan]

/assign @shashwathi

[evankanderson]

/kind cleanup

[tcnghia]

Knative Serving now can work fine without sidecar injection, so I am closing this issue.