PodSpec should be considered Buildable in all extensions

[snowdrop-bot]

The issue is that PodSpec which is part of the core model, is also used in extensions (e.g. knative).
By not considering it Buidlable, visitors are broken causing lots of issues in downstream projects.

A manifestation of the issue: quarkusio/quarkus#16376

So, PodSpec needs to be included.
Ideally, we should be more to be more generic and somehow include all core types.

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fabric8io#2976

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$upstream:2976$