Configuration option to specify Messenger transport

[loevgaard]

With the current implementation I have to manually add your commands to the messenger routing config to use an async transport. Instead it would be nice to have a configuration option named transport or something. This transport could then be used to route your messages.

I have done something similar here (although I think it's ugly): https://github.com/Setono/SyliusFragmentTranslationPlugin/blob/master/src/DependencyInjection/SetonoSyliusFragmentTranslationExtension.php#L36-L53

[loevgaard]

I am not sure this is the better approach now. I like installation instructions like this: https://github.com/Setono/SyliusFeedPlugin#step-6-using-asynchronous-transport-optional-but-recommended