use runsc shim at ateom-gvisor

[milantracy]

ateom-gvisor at cmd/ateom-gvisor currently starts sandbox using runsc (an OCI compatible runtime) directly, I would like to move substrate to use runsc shim.

we can potentially get benefits on lifecycle management by doing the migration, The shim daemon manages runsc child processes (sandbox, gofer) and reaps them automatically. Right now ateom does this itself with go-reap and reapLock coordination around every exec.Command.

And we can introduce a runtime-agnostic interface that we can introduce to manage other runtime as well, for example, #123

[milantracy]

There will be 2 phases for this migration:

phase 1: Upstream gVisor changes

• we will define service proto that expose checkpoint and restore ttrpc service via runsc shim
• the new ttrpc service will be registered at runsc shim along with existing task.RegisterTaskService (for containerd)
  • containerd will call ttrpc at containerd.task.v2.Task
  • ateom-gvisor will call ttrpc at its own name, e.g. gvisor.shim.v1.TaskServiceExt

phase 2: substrate changes

• extra a runtime agnostic interface that manages sandbox from ateom-gvisor
• implement the interface by update the existing methods in ateom-gvisor
• replacing runsc implementation with runsc-shim implementation by invoking ttrpc requests