cli: Add empty neofs-cli app structure

In the following release `neofs-cli` will be used to directly manage NeoFS Node.
All required definitions and interfaces are also moving from `neofs-api` to
`neofs-node` repository, so it's more convinient to have `neofs-cli` here.

Signed-off-by: Stanislav Bogatyrev <stanislav@nspcc.ru>

cmd/neofs-cli/modules/root.go

@@ -0,0 +1,74 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/spf13/cobra"
+
+	homedir "github.com/mitchellh/go-homedir"
+	"github.com/spf13/viper"
+)
+
+var cfgFile string
+
+// rootCmd represents the base command when called without any subcommands
+var rootCmd = &cobra.Command{
+	Use:   "neofs-cli",
+	Short: "Command Line Tool to work with NeoFS",
+	Long: `NeoFS CLI provides all basic interactions with NeoFS and it's services.
+
+It contains commands for interaction with NeoFS nodes using different versions
+of neofs-api and some useful utilities for compiling ACL rules from JSON
+notation, managing container access through protocol gates, querying network map
+and much more!`,
+}
+
+// Execute adds all child commands to the root command and sets flags appropriately.
+// This is called by main.main(). It only needs to happen once to the rootCmd.
+func Execute() {
+	if err := rootCmd.Execute(); err != nil {
+		fmt.Println(err)
+		os.Exit(1)
+	}
+}
+
+func init() {
+	cobra.OnInitialize(initConfig)
+
+	// Here you will define your flags and configuration settings.
+	// Cobra supports persistent flags, which, if defined here,
+	// will be global for your application.
+
+	rootCmd.PersistentFlags().StringVar(&cfgFile, "config", "", "config file (default is $HOME/.config/neofs-cli/config.yaml)")
+
+	// Cobra also supports local flags, which will only run
+	// when this action is called directly.
+	// rootCmd.Flags().BoolP("toggle", "t", false, "Help message for toggle")
+}
+
+// initConfig reads in config file and ENV variables if set.
+func initConfig() {
+	if cfgFile != "" {
+		// Use config file from the flag.
+		viper.SetConfigFile(cfgFile)
+	} else {
+		// Find home directory.
+		home, err := homedir.Dir()
+		if err != nil {
+			fmt.Println(err)
+			os.Exit(1)
+		}
+
+		// Search config in home directory with name ".main" (without extension).
+		viper.AddConfigPath(home)
+		viper.SetConfigName(".config/neofs-cli")
+	}
+
+	viper.AutomaticEnv() // read in environment variables that match
+
+	// If a config file is found, read it in.
+	if err := viper.ReadInConfig(); err == nil {
+		fmt.Println("Using config file:", viper.ConfigFileUsed())
+	}
+}