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]

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. https://github.com/containerd/nerdctl/pull/524

[fahedouch]

thanks @AkihiroSuda