Support multi-level nested command groups

[dblevins]

Summary

Add support for multi-level nested command groups so that a single @Command class can define commands at multiple depths in the command tree. Today, Crest supports one level of grouping (@Command("quote") on a class, @Command("create") on a method), but not deeper nesting like quote line-item add.

Motivation

CLI tools like az, gcloud, and kubectl use multi-level command groups extensively:

az storage blob upload
kubectl config set-context
gcloud compute instances create

In Crest-based CLIs, we currently flatten these into hyphenated names:

sforce quote add-line-item TT-00012345 ...

When we would prefer:

sforce quote line-item add TT-00012345 ...

Requirements

• A command class should be able to have both direct subcommands and nested groups at the same level:

  quote create <args>          # direct subcommand
  quote line-item add <args>   # nested group
  quote line-item list <args>  # nested group
  quote line-item remove <id>  # nested group
  

• This must be mix-and-match -- any node in the command tree can have both leaf commands and further sub-groups.

• Help output should reflect the hierarchy, showing groups and commands at each level.

Possible Approach

One possible direction is allowing @Command on inner classes to define sub-groups:

@Command("quote")
public class QuoteCommand {

    @Command
    public void create(...) { }

    @Command
    public void delete(...) { }

    @Command("line-item")
    public class LineItemCommand {

        @Command
        public void add(...) { }

        @Command
        public void list(...) { }

        @Command
        public void remove(...) { }
    }
}

This is just one option -- the implementation design is open.

[rmannibucau]

One trivial option can be to move the value of @Command to an array of string, will get both this feature doable and the flexibility of the modularity of hte code - even plugins ;)