[k8s] Define `k8s.pod.hostname`, `k8s.pod.ip`, `k8s.pod.start_time` as Semantic Conventions

[ChrsMark]

Area(s)

area:k8s

Propose new conventions

As highlighted by #3120, k8s.pod.hostname, k8s.pod.ip and k8s.pod.start_time are not defined as Semantic Conventions yet.

This issue tracks the progress around this. Specifically this is a prerequisite for #3120 and the stability of the k8sattributes processor of the Collector: open-telemetry/opentelemetry-collector-contrib#44483

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[ChrsMark]

For k8s.pod.ip supersedes #1160. From that older issue it seems the k8s.pod.ip attribute can be added now as a descriptive attribute to the Pod entity.