helm-chart/ray-cluster: Allow setting pod lifecycle

[ulfox]

In this commit we extend the capability of ray-cluster helm chart to allow adding pod lifecycle in the head & worker nodes

Why are these changes needed?

The changes are not needed, but can extend the customization of the head & worker pods.

We can use postStart and preStop lifecycles to run custom commands. For example, send a webhook after a headnode starts.

Example:

  lifecycle:
    preStop:
      exec:
        command: ["/bin/sh","-c","ray stop"]

[DmitriGekhtman]

Thanks!

This PR makes sense. Would you mind documenting the newly added field in values.yaml?

On a related note,
all of the sample RayCluster CR yamls specify a pre-stop "ray stop" hook.
In the future, we might have the operator auto-configure this hook (with preference for a user-specified overrides).

[DmitriGekhtman]

This change looks good to me -- we can think later about whether to document it in values.yaml, as most users shouldn't have to touch this setting.