Support isolated OIDC token cache directories in generated `kubeconfig`s to prevent token conflicts when switching between users/clusters. Configurable via server flags and omnictl `--oidc-cache-base-dir` `--oidc-cache-isolation`.
Also upgrade exec credential API to v1 and add interactiveMode field.
Signed-off-by: Oguz Kilcan <oguz.kilcan@siderolabs.com>