refactor: Make --context cli flag a global option (#174)

* Make `--context` flag a global option and remove duplication

* Remove unused dns showOptions struct

* First pass of refactoring "active context" determination and validation

* Undo some refactoring in cli.go to keep diff more focused

cmd/uncloud/dns/show.go

@@ -10,32 +10,21 @@ import (
 	"github.com/spf13/cobra"
 )
 
-type showOptions struct {
-	context string
-}
-
 func NewShowCommand() *cobra.Command {
-	opts := showOptions{}
-
 	cmd := &cobra.Command{
 		Use:   "show",
 		Short: "Print the cluster domain name.",
 		RunE: func(cmd *cobra.Command, args []string) error {
 			uncli := cmd.Context().Value("cli").(*cli.CLI)
-			return show(cmd.Context(), uncli, opts)
+			return show(cmd.Context(), uncli)
 		},
 	}
 
-	cmd.Flags().StringVarP(
-		&opts.context, "context", "c", "",
-		"Name of the cluster context. (default is the current context)",
-	)
-
 	return cmd
 }
 
-func show(ctx context.Context, uncli *cli.CLI, opts showOptions) error {
-	clusterClient, err := uncli.ConnectCluster(ctx, opts.context)
+func show(ctx context.Context, uncli *cli.CLI) error {
+	clusterClient, err := uncli.ConnectCluster(ctx)
 	if err != nil {
 		return fmt.Errorf("connect to cluster: %w", err)
 	}