Implement a check-for-upgrade flag in the kubectl plugin

[johscheuer]

What would you like to be added/changed?

We should implement the functionality that our plugin can check if a newer version exists by simply checking the latest release (see: https://github.com/FoundationDB/fdb-kubernetes-operator/tree/main/kubectl-fdb#installation). This should be either a new sub command or a flag in the version sub command.