Add a core package to avoid hiding tasks in the docs

[fdelbrayelle]

Describe the issue

Docs don't support to have both tasks/triggers in the root package and in a sub package, whether it's:

• all tasks/triggers in the root package
• only tasks/triggers in sub packages

1. That's why these tasks/triggers should be moved (including the packages examples to update & tests if existing) to io.kestra.plugin.kubernetes.core:

• plugin-kubernetes/src/main/java/io/kestra/plugin/kubernetes/PodCreate.java

2. Add the missing package-info.java in the kubectl package.

Environment

• Kestra Version: develop

[Malaydewangan09]

This might be a nice issue for first-time contributors!