cgroups v2 support about singularity

[dtrudg]

We use a TOML cgroups config format that is defined here: https://github.com/sylabs/singularity/blob/master/internal/pkg/cgroups/config_linux.go

This is closely related to the opencontainers/runtime-spec/specs-go constructs: https://github.com/opencontainers/runtime-spec/blob/master/specs-go/config.go

At present, the v1 cgroups config is applied via containerd/cgroups routines that directly accept LinuxResources (s below):

singularity/internal/pkg/cgroups/cgroups_linux.go

Line 79 in 1fbbe63

m.cgroup, err = cgroups.New(cgroups.V1, path, s)

For cgroups v2 a v2.LoadManager followed by a v2.NewManager call will setup resource limits. The resources type is now v2.Resources (https://github.com/imilchev/cgroups/blob/dd572fc860afa53ef84ec0a8cd040b3e86f78bb4/v2/manager.go#L59) - we cannot simply pass the specs-go LinuxResources type, even though it has had a 'Unified' attribute added to support representing v2.

We need to investigate a way of moving forward... how the existing toml structure maps into the v2.Resources etc.