25307: release-2.0: importccl: check node health and compatibility during IMPORT planning r=mjibson a=mjibson

Backport 1/1 commits from #25162.

/cc @cockroachdb/release

---

Simplify the LoadCSV signature by taking just a PlanHookState for any
argument that can be fetched from it. Determine the node list using
this new health check function. We can remove the rand.Shuffle call
because the map iteration should produce some level of randomness.

Fixes #12876

Release note (bug fix): Fix problems with imports sometimes failing
after node decommissioning.


Co-authored-by: Matt Jibson <matt.jibson@gmail.com>