Add preflight checks for existence of tanzu binary with tanzu context get-token command while generating kuebconfig for UCP context

[prkalle]

Describe the feature request
tanzu CLI generates the kubeconfig with Exec plugin for UCP context in order to fetch the CSP token dynamically. However this kubeconfig expects the tanzu binary with the tanzu context get-token command support to be present in user's PATH.

So, CLI while generating the Kubeconfig for UCP context, it should consider adding a preflight check (may be checking for tanzu context get-token command) before assigning this command as Exec plugin in the kubeconfig

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