Support shell completion

[timebertt]

The plugin should support shell completion to make it easier to use.
This will make kubectl power users feel right at home.

[timebertt]

For the plugin to provide shell completion, it needs to add a kubectl_complete-revisions executable on the PATH, see kubernetes/kubernetes#105867 (also see sample-cli-plugin).
The script could be as simple as:

#!/usr/bin/env bash

# Call cobra's built-in __complete command passing all arguments
kubectl revisions __complete "$@"

This is not natively supported by krew right now: kubernetes-sigs/krew#812
In the meantime, https://github.com/marckhouzam/kubectl-plugin_completion can generate a script for this plugin to enable completion.

The revisions plugin should

• add a description on how to enable shell completion
• implement cobra's mechanism for completing arguments (ValidArgsFunction and friends)