[RfC] generalize support for custom extensions

[ndeloof]

Is your feature request related to a problem? Please describe.
Compose file format is used to define applications for platforms it was not initialy designed for.

• Kubernetes: with https://github.com/kubernetes/kompose and https://github.com/docker/compose-on-kubernetes
• Amazon ECS: https://docs.aws.amazon.com/AmazonECS/latest/developerguide/cmd-ecs-cli-compose-parameters.html

While the abstract model of Compose file can be adapted for such platforms, they might miss some specific configuration attributes. To workaround this limitation, a sibling config file might be necessary (see for example https://docs.aws.amazon.com/AmazonECS/latest/developerguide/cmd-ecs-cli-compose-ecsparams.html)

Compose file format do support x-* extension fields. Docker cli use such extension for docker stack deploy command to support kubernetes-specific configuration attributes (pull-policy, pull-secrets, service-type).

But x-* fields are restricted to root and top-level elements only. Platform specific extension should be allowed anywhere, so one can declare custom fields where it makes much sense within Compose model.

Also, x-* are not platform-scoped, which means Platforms A and B could both support some x-magic extension but with a very different effect.

Describe the solution you'd like

1. Allow a Compose file to use x-* extensions everywhere
2. Recommend use of vendor prefixes, like Docker cli does with x-kubernetes*

Bonus point : this allows more experimentation with Compose file format, and comparable extensions supported by a significant set of platforms could then be adopted by next iteration of the Compose file format. (the same way CSS browser extensions get adopted)

Describe alternatives you've considered

Additional context