Extend kubectl-operator to install and uninstall ClusterExtensions

[ankitathomas]

Extend kubectl-operator to

• install ClusterExtensions : create a ClusterExtension for a package using an existing ServiceAccount, with relevant optional constraints (version, channel etc.)
• uninstall ClusterExtensions : remove the ClusterExtension for a package

[ankitathomas]

Done as of operator-framework/kubectl-operator#228