Discover the target pod by labels

If we want to collect the pod logs, the simple way is to add a label for this pod. such as:

```yaml
labels:
vector.oam.dev/std-collector: enabled
```

Currently, we must create a CR.

Start by reading the Go controller's current CR-based pod log collection path and compare it with the proposed vector.oam.dev/std-collector label. Define how labeled pods are discovered and how the existing CR flow is affected; done means labeled pods can have their logs collected without creating a CR, with appropriate coverage for the controller behavior.

Written by the indexing model from the issue text.