Rename pod directive to podOptions

[bentsherman]

This PR renames the pod directive to podOptions, for consistency with clusterOptions, containerOptions, etc. Nextflow will still accept pod but will print a deprecation warning.

[pditommaso]

I will the idea, I'd resist making this change only for naming uniformity. I guess there's a lot of config out there that would be impacted and can be annoying to update them.

I would keep this possibility if at some point there's need to change the pod configuration structure or keep in sync with the config re-engineering #2723

[bentsherman]

To be clear, this PR only deprecates the pod option, so pod is still accepted but users get a deprecation warning. Anyway, this PR is here if you want to merge it. Although I left it as a draft because I wanted to make sure I did the deprecation correctly. Still need to test locally.

[pditommaso]

Understand that, but still think we can keep this for when it may be needed to re-organise the K8s config. Also considering the discussion to use jobs instead of pods #2751, does still make sense to use podOptions? maybe it could be better to use k8sOptions instead.

Closing this for now.

[bentsherman]

I think podOptions still makes sense because it refers to the pod-specific options, whether the pod is standalone or part of a job. k8sOptions would be confusing with the k8s config options, we would want to rethink how the k8s options are organized overall.