gsi_util: adding subcommand framework

With this CL, new command can be easily added:
  1. Add a new file: gsi/gsi_util/gsi_util/commands/hello.py
     And implements a function to register parsers into gsi_util's main parser.

     def setup_command_args(subparsers)

  2. In gsi/gsi_util/gsi_util.py, add the new command into GsiUtil():
     _COMMANDS = ['hello']

Bug: 70588453
Test: make gsi_util
Test: gsi_util --help
Test: gsi_util hello --foo
Test: gsi_util --debug hello --bar

Change-Id: I54fa6d1dea5afed724d9ac4a99f88bf78da82732