feat: implement kube_pod_status_initialized_time

[opeco17]

What this PR does / why we need it:

This PR enables kube-state-metrics to provide the time when a pod is initialized in the lifecycle.

This metric is useful to calculate how long it takes to complete startup probe.

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

increases

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 #2147

[k8s-ci-robot]

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[dgrisonnet]

Thank you for the contribution @opeco17 :)

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dgrisonnet, opeco17

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