[BUG] transferEnv doesn't seem to work with initContainers

[BryanDollery]

What happened:
My init container doesn't get the transferred environment from the main container

What you expected to happen:
I expected the init container to get the environment

How to reproduce it (as minimally and precisely as possible):

apiVersion: apps.kruise.io/v1alpha1
kind: SidecarSet
metadata:
  name: event-registration-sidecarset
spec:
  selector:
    matchLabels:
      sidecar: registrar
  initContainers:
    - name: registrar
      image: registry.labs:5000/labs/event-registration-sidecar
      transferEnv:
        - sourceContainerName: main
          envName: NAMESPACE
        - sourceContainerName: main
          envName: SERVICE
        - sourceContainerName: main
          envName: VERSION
        - sourceContainerName: main
          envName: EVENT_REGISTRATION
      env:
        - name: REGISTRATION_CHANNEL
          valueFrom:
            configMapKeyRef:
              name: control-config
              key: registration

Anything else we need to know?:
I guess this is because the main container hasn't been created at the time the init container runs and so can't take the environment directly from it

Environment:
Nothing special

• Kruise version:
  openkruise/kruise-manager:v1.0.0

• Kubernetes version (use kubectl version):
  Client Version: version.Info{Major:"1", Minor:"22", GitVersion:"v1.22.4", GitCommit:"b695d79d4f967c403a96986f1750a35eb75e75f1", GitTreeState:"clean", BuildDate:"2021-11-17T15:48:33Z", GoVersion:"go1.16.10", Compiler:"gc", Platform:"linux/amd64"}
  Server Version: version.Info{Major:"1", Minor:"22", GitVersion:"v1.22.5+k3s1", GitCommit:"405bf79da97831749733ad99842da638c8ee4802", GitTreeState:"clean", BuildDate:"2021-12-18T00:43:30Z", GoVersion:"go1.16.10", Compiler:"gc", Platform:"linux/amd64"}

• Install details (e.g. helm install args):
  kruise installed through official chart with no extra arguments

• Others:

[FillZpp]

Yep, it seems transferEnv and some other fields like shareVolumePolicy are not supported for initContainers yet.
https://github.com/openkruise/kruise/blob/master/pkg/webhook/pod/mutating/sidecarset.go#L267

@BryanDollery Are you interested in approving this? Or you may ask @zmberg to help you with it.

[pigletfly]

I'd like to take a shot on this.

[pigletfly]

/assign

[FillZpp]

Closed by #897
Good job!