add project configration inheritance support

[Jnoack331]

This would help with configuration managment because you can extend another .shopware-project.yml file to reuse or override configurations. This is useful when you have multiple projects with the same configuration. You can also use this to create a base configuration for your stages or teams and extend it for your own needs.
This also can be used to toggle specific plugin configurations for different stages e.g. enabling/disabling the Paypal sandbox mode depending on the environment.

[shyim]

What do you think about naming it include and allow to be an array of files? In that way we would support multiple files.

Little bit like docker compose https://docs.docker.com/compose/multiple-compose-files/include/

[Jnoack331]

@shyim sure, i guess the priority would be set by the order of the files provided?

[shyim]

yes :)

[Jnoack331]

@shyim Just looked a bit into the docker documentation. That include change would reverse the override logic correct? Would that mean an include could override a value in the current configuration, or is this handled differently in docker compose?

[Jnoack331]

@shyim I changed it so its now using the include statement with multiple files to include. I didnt change the base merge logic so that values from the main file always have priority, this is propably the most intuitive.

[shyim]

Thank you!