add tink-cli completion subcommand

Signed-off-by: Marques Johansson <marques@packet.com>

.gitignore

@@ -4,6 +4,7 @@ cmd/tink-cli/tink-cli
 cmd/tink-server/tink-server
 cmd/tink-worker/tink-worker
 .idea
+.vscode
 
 # Terraform
 .terraform

cmd/tink-cli/.gitignore

@@ -1 +1,2 @@
 /tink
+.vscode

cmd/tink-cli/cmd/completion.go

@@ -0,0 +1,61 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/spf13/cobra"
+)
+
+// completionCmd returns the completion command that, when run, generates a
+// bash or zsh completion script for the CLI
+func completionCmd(name string) *cobra.Command {
+	return &cobra.Command{
+		Use:   "completion [bash|zsh|fish|powershell]",
+		Short: "Generate completion script",
+		Long: `To load completions:
+	Bash:
+	$ source <(tink-cli completion bash)
+	Bash (3.2.x):
+	$ eval "$(tink-cli completion bash)"
+	# To load completions for each session, execute once:
+	Linux:
+	  $ tink-cli completion bash > /etc/bash_completion.d/tink-cli
+	MacOS:
+	  $ tink-cli completion bash > /usr/local/etc/bash_completion.d/tink-cli
+	Zsh:
+	$ source <(tink-cli completion zsh)
+	# To load completions for each session, execute once:
+	$ tink-cli completion zsh > "${fpath[1]}/_tink-cli"
+	Fish:
+	$ tink-cli completion fish | source
+	# To load completions for each session, execute once:
+	$ tink-cli completion fish > ~/.config/fish/completions/tink-cli.fish
+	`,
+		DisableFlagsInUseLine: true,
+		Hidden:                true,
+		ValidArgs:             []string{"bash", "zsh", "fish", "powershell"},
+		Args:                  cobra.ExactValidArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			switch args[0] {
+			case "bash":
+				return cmd.Root().GenBashCompletion(os.Stdout)
+			case "zsh":
+				return cmd.Root().GenZshCompletion(os.Stdout)
+			case "fish":
+				return cmd.Root().GenFishCompletion(os.Stdout, true)
+			case "powershell":
+				return cmd.Root().GenPowerShellCompletion(os.Stdout)
+			}
+			// ValidArgs make this error response dead-code
+			return fmt.Errorf("unknown shell: %q", args[0])
+		},
+		PersistentPreRunE: func(cmd *cobra.Command, args []string) error {
+			return nil
+		},
+	}
+}
+
+func init() {
+	rootCmd.AddCommand(completionCmd(rootCmd.CalledAs()))
+}