feat: add ability to override usage text of default help command

[Maks1mS]

What type of PR is this?

• feature

What this PR does / why we need it:

This PR introduces the ability to override the usage text of the default help command.

---

In v2, I implemented in my app translations for the help command by overriding the usage text after app.Setup():

func Localize(app *cli.App) {
	app.Setup()
	// ...
	for _, cmd := range app.Commands {
		if cmd.Name == "help" {
			cmd.Usage = gotext.Get("Shows a list of commands or help for one command")
		}
	}
}

However, v3 removed the cli.App and Setup method, making this approach impossible.
This PR adds a way to customize help command text, enabling use cases like localization.

Which issue(s) this PR fixes:

N/A

Release Notes

Feature: add ability to override usage text of default help command

[dearchap]

@Maks1mS can you fix the gofmt error check ?

[GwynethLlewelyn]

Cool! What exactly needs to be called (and how) to do the override?