Simplify the application run/stop cycle

[facundobatista]

The lib should provide a way to handle the application bootstrapping and finishing (in several ways: ok, error, etc).

The code is already in Charmcraft, we need to move it in here.

[facundobatista]

More thinking about this.

I think we should create a context manager, something to be used like:

with craft_cli.app_wrapper(...) as dispatcher:
    dispatcher.pre_parse_args(sys.argv[1:])
    dispatcher.load_command(None)
    dispatcher.run()
sys.exit(dispatcher.return_code)

The app_wrapper signature would be:

def app_wrapper(
    app_name: str, 
    greeting_msg: str, 
    command_groups: List[CommandGroup], 
    *, 
    summary: str = "",
    init_emitter_mode: Optional[EmitterMode] = EmitterMode.NORMAL, 
    log_filepath: Optional[pathlib.Path] = None,
    extra_global_args: Optional[List[GlobalArgument]] = None,
)

The context manager would:

• init emit with proper values
• instantiate Dispatcher with proper values and return it as the object to be used
• "hide" the handling of specific ArgumentParsingError and ProvideHelpException; for these both cases it would automatically show messages and cut further execution, leaving dispatcher.return_code with the proper value
• also hide catching CraftError, KeyboardInterrupt and generic Exception, doing the right thing in each case
• provide a simpler and direct to use return code

Improvements:

• Much simpler to use
• Less error prone as no comple structure is needed just for the lib to work
• No need to expose internal exceptions in the simple usage at least: ArgumentParsingError should remain public because it's handy for the command to expose a complicated argument parsing failure, but it doesn't need to be presented to the developer the first time she reads about this lib; ProvideHelpException could be made private (OTOH is ok to leave it public if the developer needs to fallback into a more "manual" usage of the lib because of advance needs).
• No more manual "print"s