IBX-11795: Added worker in default configuration to handle messenger queue

[Sztig]

🎫 Issue	IBX-11795

Description:

workers:
    messenger:
        disk: 256
        commands:
            start: php bin/console messenger:consume ibexa.messenger.transport --bus=ibexa.messenger.bus --limit=100 --time-limit=60 --memory-limit=256M

I have added this worker in default .platform.app.yaml configuration to handle the ibexa/messenger queue.
Disk is specified to limit the container size as by default it will try to run on a same size as the app making it very easy to run out of memory during build.
I am only adding this to ibexa/cloud and not ibexa/post-install package as by default we are not including ibexa/messenger in 4.6 installations.

Documentation:

I think we need a section in our documentation describing this config change when installing ibexa/messenger - https://doc.ibexa.co/en/4.6/infrastructure_and_maintenance/background_tasks

[tomaszszopinski]

QA approved on Ibexa DXP 5.0 headless (local and upsun).