Support TryAddFlagSet and EnvVars and AddFlag

[robberphex]

1. Add func TryAddFlag{,Set}, which add flag{Set} and ignore conflicts.
2. Support EnvVars, and show env name in help message.
3. Support AddFlag.
   originally, AddFlag can accept a pflag.Flag, but user cannot build Value field for pflag.Flag. So I rename pflag.newStringValue to pflag.NewStringValue to make AddFlag usable, for example:

  child.Flags().AddFlag(
  	&pflag.Flag{
  		Name:     "testArg",
  		Usage:    "usage",
  		EnvVars:  []string{"TEST_ARG", "TEST_ARG1"},
  		Value:    pflag.NewStringValue("default", new(string)),
  	},
  )

[fahedouch]

@robberphex

It looks like it is not showing correctly the default value

-l, --leaderElection        Use leader election for the conroller manager. (default ) [$LEADER_ELECTION]

Extract from my source code

	leaderElection := new(bool)
	rootCmd.PersistentFlags().AddFlag(&pflag.Flag{
		Name:      "leaderElection",
		Shorthand: "l",
		Usage:     "Use leader election for the conroller manager.",
		EnvVars:   []string{"LEADER_ELECTION"},
		Value:     pflag.NewBoolValue(false, leaderElection),
	}

[AkihiroSuda]

When the both the flag value and the env var are specified, the flag value should be prioritized over the env value

containerd/nerdctl#495

[AkihiroSuda]

Could you update the commit messages to be more descriptive?

[fahedouch]

@robberphex do you plan to move forward on this?

[AkihiroSuda]

nerdctl now uses unforked cobra, so this PR is no longer needed from nerdctl perspective.
containerd/nerdctl#524

[fahedouch]

thanks @AkihiroSuda