Create a Command to Validate Cluster State

We often encounter scenarios where a user has created a cluster but for one reason or another it is not fully formed. Sometimes this is because a component failed to install, other times this is because a user removed a core component.

When supporting users, it would be very helpful to have a command we could ask them to run that validates the state of their cluster.

We already have the following command:
```
eksctl anywhere exp validate create cluster -f
```

The above command wires in all existing create cluster validations.

I propose adding a new experimental command:
```
eksctl anywhere exp validate upgrade cluster -f --kubeconfig
```

This could use all the existing upgrade cluster validations that already exist. It would just be a matter of wiring them in.

Some example invocations:
```
eksctl anywhere exp validate upgrade cluster -f mgmt.yaml

eksctl anywhere exp validate upgrade cluster -f workerload.yaml --kubeconfig mgmt/mgmt.kubeconfig
```

This is the lowest lift way I can think of to create a validation command.

Start by tracing the existing `eksctl anywhere exp validate create cluster` and upgrade-cluster command entry points, then inspect how their existing validations are wired. Confirm the proposed `validate upgrade cluster` invocations work with and without `--kubeconfig`, and that they run the existing upgrade validations against the supplied cluster configuration.

Written by the indexing model from the issue text.