EKS Anywhere should validate that user-provided taints do not have duplicate keys

Kubernetes does not support taints with duplicate keys on the same nodes.

Attempting to join a node with duplicate taint keys results in an error.

EKS Anywhere does not validate that user-provided taints have distinct keys.

A validation should be added to ensure that taints provided in their the CP config or WN config have distinct keys.

The issue does not name specific files or tests. Locate the validation paths for taints in the control-plane and worker-node configurations, then add coverage for duplicate keys and confirm that invalid configurations are rejected before node join.

Written by the indexing model from the issue text.