Copy labels onto the resources created by the request-cert process

It'd be easier to clean up all the resources created by us if we were to put labels on the CSRs and Secrets when we create them. Currently [our docs have to instruct users to clean those up separately from everything else](https://www.cockroachlabs.com/docs/stable/orchestrate-cockroachdb-with-kubernetes.html#step-10-stop-the-cluster) because they don't have the expected labels.

It should be easy enough to [use the Kubernetes downward API to get the pod's labels](https://kubernetes.io/docs/tasks/inject-data-application/downward-api-volume-expose-pod-information/#store-pod-fields) from within the request-cert init container, then copy all of them verbatim into all CSRs and Secrets we create.

This would make a good starter task for anyone interested in playing around with kubernetes.

cc @mberhault @bobvawter

Locate the request-cert init container and the paths where it creates CSRs and Secrets. Read the Kubernetes downward API guidance linked in the issue, then verify that all created resources receive the pod labels verbatim and that the cleanup workflow no longer needs to treat them separately.

Written by the indexing model from the issue text.