CLI: ability to load command handlers from the available in the model extensions

At the moment, the CLI supports Java ServiceLoader mechanism to load command handlers from outside the CLI module during the initialization. Although, for that to work, an optional dependency on the module which is providing its commands has to be added to the CLI module.xml.

This enhancement is created to remove the requirement to modify the CLI module.xml to load external commands. Instead, the CLI could consult the list of available extensions and determine the list of modules to look for command handler in.