feat: add start and termination timestamps for init containers

[drolando]

What this PR does / why we need it:
The new Sidecar Containers introduced in k8s 1.28 implement sidecars as init containers: https://kubernetes.io/docs/concepts/workloads/pods/sidecar-containers .

I wanted to monitor when each native sidecar starts and ends, however kube-state-metrics only emits those metrics for normal containers.

Given that these are new metrics I've set them as ALPHA, let me know if you don't agree

How does this change affect the cardinality of KSM: (increases, decreases or does not change cardinality)

The cardinality increases since these metrics will be emitted also for init containers.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #2627

[linux-foundation-easycla]

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[CatherineF-dev]

Overall LGTM. Need to fix two small tests.

• https://github.com/kubernetes/kube-state-metrics/actions/runs/14071399444/job/39406603122?pr=2638
• https://github.com/kubernetes/kube-state-metrics/actions/runs/14071399444/job/39406604045?pr=2638