Resource attribute validation using data source outputs

While using the Terraform plugin framework for development. What are the ways to validate an attribute at the plan phase? I believe there are custom validators but not sure if then can access the ProviderData to query the infrastructure backend.

resource "myk8s_cluster" "mycluster" {
  name     = "mycluster"
  region   = "us-central" # To be validated using `regions` data source

data "myk8s_regions" "regions" {

output "regionslist" {
  value =