Improve and unify plugin support

Current situation

• Channel handlers receive all ExecutionCommand messages, even the ones that do not target them.
• Some plugin handler should return a status objects and other should return nothing.

Desired outcome

• Filter out ExecutionCommand messages that target another channel handler (a new args has been added to make_plugin).
• Channel handlers should return nothing.
• Generator handler should have a debug log when called.
• Generator handler should use the inputs helpers.
• Provider and Generator handlers should use an outputs helper.