Extend kubectl-operator plugin to support ClusterExtension

[ankitathomas]

Extend kubectl plugin to support commands for ClusterExtensions including:

• list : list currently installed ClusterExtensions with useful information including status
• get : get info on an existing ClusterExtension
• install : create a ClusterExtension for a package using an existing ServiceAccount, with relevant optional constraints (version, channel etc.)
• uninstall : remove the ClusterExtension for a package
• upgrade : update the ClusterExtension for a package for a provided version/channel etc.
• list-available : list bundles available to be installed with support for relevant constraints (package, channel, version range/version, catalog label selectors etc.)