Support label or annotate the pod to indicate that Kmesh is in charge of traffic management.

**What would you like to be added**:
Support label or annotate the pod to indicate that Kmesh is in charge of traffic management.

**Why is this needed**:
Kmesh supports collaborating with existing mesh data plane. Consider the following scenario: a namespace has already been injected with a sidecar, and then the Kmesh data plane is injected into this namespace.

According to the design, existing pods in the namespace will continue to have their traffic governed by the sidecar. However, the traffic of new pods created in the namespace will be taken over by Kmesh, although a new pod will still have a sidecar created, as shown in the diagram.

In this situation, there needs to be a method to inform the operations team which pods' traffic is being managed by Kmesh; otherwise, confusion may arise.

No files, tests, or entry points are identified. Start by locating the Kubernetes pod injection and traffic-management paths, then determine how to distinguish existing sidecar-managed pods from new Kmesh-managed pods and define the label or annotation that operations teams should see.

Written by the indexing model from the issue text.