Add shell completion support to plugin example

The use of `kubectl_complete-ns` enables shell completion when calling
the plugin through `kubectl ns`.  The script only completes flag names
and namespace arguments.

Signed-off-by: Marc Khouzam <marc.khouzam@gmail.com>

staging/src/k8s.io/sample-cli-plugin/README.md

@@ -58,6 +58,29 @@ that kubectl points to.
 It can also be used as a means of showcasing usage of the cli-runtime set of utilities to aid in
 third-party plugin development.
 
+## Shell completion
+
+This plugin supports shell completion when used through kubectl.  To enable shell completion for the plugin
+you must copy the file `./kubectl_complete-ns` somewhere on `$PATH` and give it executable permissions.
+
+The `./kubectl_complete-ns` script shows a hybrid approach to providing completions:
+1. it uses the builtin `__complete` command provided by [Cobra](https://github.com/spf13/cobra) for flags
+1. it calls `kubectl` to obtain the list of namespaces to complete arguments (note that a more elegant approach would be to have the `kubectl-ns` program itself provide completion of arguments by implementing Cobra's `ValidArgsFunction` to fetch the list of namespaces, but it would then be a less varied example)
+
+One can then do things like:
+```
+$ kubectl ns <TAB>
+default          kube-node-lease  kube-public      kube-system
+
+$ kubectl ns --<TAB>
+--as                        -- Username to impersonate for the operation. User could be a regular user or a service account in a namespace.
+--as-group                  -- Group to impersonate for the operation, this flag can be repeated to specify multiple groups.
+--as-uid                    -- UID to impersonate for the operation.
+--cache-dir                 -- Default cache directory
+[...]
+```
+
+Note: kubectl v1.26 or higher is required for shell completion to work for plugins.
 ## Cleanup
 
 You can "uninstall" this plugin from kubectl by simply removing it from your PATH:

staging/src/k8s.io/sample-cli-plugin/kubectl_complete-ns

@@ -0,0 +1,24 @@
+#!/usr/bin/env bash
+
+# If we are completing a flag, use Cobra's builtin completion system.
+# To know if we are completing a flag we need the last argument starts with a `-` and does not contain an `=`
+args=("$@")
+lastArg=${args[((${#args[@]}-1))]}
+if [[ "$lastArg" == -* ]]; then
+   if [[ "$lastArg" != *=* ]]; then
+      kubectl ns __complete "$@"
+   fi
+else
+   # TODO Make sure we are not completing the value of a flag.
+   # TODO Only complete a single argument.
+   # Both are pretty hard to do in a shell script.  The better way to do this would be to let
+   # Cobra do all the completions by using `cobra.ValidArgsFunction` in the program.
+   # But the below, although imperfect, is a nice example for plugins that don't use Cobra.
+
+   # We are probably completing an argument.  This plugin only accepts namespaces, let's fetch them.
+   kubectl get namespaces --output go-template='{{ range .items }}{{ .metadata.name }}{{"\n"}}{{ end }}'
+
+   # Turn off file completion.  See the ShellCompDirective documentation within
+   # https://github.com/spf13/cobra/blob/main/shell_completions.md#completion-of-nouns
+   echo :4
+fi