Support `runtime` parameter in 3.x schema

[khimaros]

Is your feature request related to a problem? Please describe.
I am trying to upgrade my configs from 2.3 to 3.x configuration version. However, I currently use the runtime parameter to selectively enable runsc for some containers.

Describe the solution you'd like
Provide a way to specify runtime in the 3.x schema.

Describe alternatives you've considered
The only alternatives I see is remaining on 2.3 or migrating off of docker-compose for managing my runsc containers.

Additional context
It appears that some work was done on this here #6240 but it does not appear to have made its way into the 3.8 schema.

[ndeloof]

docker-compose since 1.28 adopted the Compose specification and as such supports all v2 and v3 attributes in a compose.yaml file