This PR introduces a new CLI system using

• click (the CLI framework, which we already had as transitive dependency)
• rich (for colors, emojis, and other super important stuff!)
  
• stevedore (for the plugin mechanism)
• It also moves the VERSION constant into localstack.__init__ as per convention

Plugin loading

⚠ the localstack-pro CLI now only works when installed as a distribution (using pip install or setup.py install). extensions simply residing in the python path is no longer sufficient for loading them.

to debug the plugin lifecycle run

DEBUG_PLUGINS=1 bin/localstack

Creating plugins

Plugins are advertised using setuptools entrypoints. To create a CLI plugin, first implement the LocalstackCliPlugin class that adds click commands to the localstack command group (a click.Group object):

class MyCliPlugin(LocalstackCliPlugin):
    name = "mine"

    def attach(self, cli: LocalstackCli) -> None:
        cli.group.add_command(...) # add your click commands to `localstack`

then, in the setup.py, add an entry point to the namespace localstack.plugins.cli (this will be automated in the future!)

setup(entry_points={
    "localstack.plugins.cli": [
        "mine = path.to.plugin:MyCliPlugin",
    ]
})

In localstack, we then use stevedore's Extension pattern to load extensions (using an ExtensionManager) in the namespace localstack.plugins.cli