Original PR: symfony/symfony#26803
Along with symfony/symfony#27275, it allows to easily list the available messages per bus and respective handlers.

Output samples:

• no args: https://github.com/symfony/symfony/blob/60aaa808c148f7edb63ee99ed3b41a81da0d093f/src/Symfony/Component/Messenger/Tests/Command/DebugCommandTest.php#L59-L84
• with bus name: https://github.com/symfony/symfony/blob/60aaa808c148f7edb63ee99ed3b41a81da0d093f/src/Symfony/Component/Messenger/Tests/Command/DebugCommandTest.php#L91-L108