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

[danbudris]

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.

[jiayiwang7]

Validation on Kubernetes: https://github.com/kubernetes/kubernetes/blob/master/pkg/apis/core/validation/validation.go#L4991