Automatically defer subcommand initialization for derives

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Clap Version

4.3.3

Describe your use case

#4792 added deferred command initialization but no support exists yet for the derive API.

Describe the solution you'd like

Automatically defer all non-essential subcommand calls.

Alternatives, if applicable

No response

Additional Context

Automatic deferment would be a breaking change though we could make it opt-in through the remainder of 4.x

A rough sketch

• Extend Args to have defer_augment_args
• Extend Subcommand to have defer_augment_subcommand
• New #[command(defer = <bool>)] magic attribute
  • defaults to false in clap v4
  • defaults to true in clap v5, mark it as deprecated
  • remove it in clap v6
• If defer = false on a top-level container, always call defer_augment_args along with augment_args
• If defer = true on a top-level container
  • Defer Command::subcommand and Command::arg calls
  • if defer = true on a flatten, augment_args only calls the same of its children
  • If defer = false on a flatten, defer augment_args until when defer_augment_args is called