api/services/instrospection: add PluginInfo

The new `PlunginInfo()` call can be used for instrospecting the details
of the runtime plugin.

```console
$ ctr plugins inspect-runtime --runtime=io.containerd.runc.v2 --runc-binary=runc
{
    "Name": "io.containerd.runc.v2",
    "Version": {
        "Version": "v2.0.0-beta.0-XX-gXXXXXXXXX.m",
        "Revision": "v2.0.0-beta.0-XX-gXXXXXXXXX.m"
    },
    "Options": {
        "binary_name": "runc"
    },
    "Features": {
        "ociVersionMin": "1.0.0",
        "ociVersionMax": "1.1.0-rc.2",
        ...,
    },
    "Annotations": null
}
```

The shim binary has to support `-info` flag, see `runtime/v2/README.md`

Replaces PR 8509 (`api/services/task: add RuntimeInfo()`)

Co-authored-by: Derek McGowan <derek@mcg.dev>
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

cmd/ctr/commands/commands_unix.go

@@ -19,10 +19,24 @@
 package commands
 
 import (
+	"errors"
+
+	"github.com/containerd/containerd/v2/core/runtime/v2/runc/options"
+	runtimeoptions "github.com/containerd/containerd/v2/pkg/runtimeoptions/v1"
 	"github.com/urfave/cli"
 )
 
 func init() {
+	RuntimeFlags = append(RuntimeFlags, cli.StringFlag{
+		Name:  "runc-binary",
+		Usage: "Specify runc-compatible binary",
+	}, cli.StringFlag{
+		Name:  "runc-root",
+		Usage: "Specify runc-compatible root",
+	}, cli.BoolFlag{
+		Name:  "runc-systemd-cgroup",
+		Usage: "Start runc with systemd cgroup manager",
+	})
 	ContainerFlags = append(ContainerFlags, cli.BoolFlag{
 		Name:  "rootfs",
 		Usage: "Use custom rootfs that is not managed by containerd snapshotter",
@@ -44,3 +58,42 @@ func init() {
 		Usage: "File path to a device to add to the container; or a path to a directory tree of devices to add to the container",
 	})
 }
+
+func getRuncOptions(context *cli.Context) (*options.Options, error) {
+	runtimeOpts := &options.Options{}
+	if runcBinary := context.String("runc-binary"); runcBinary != "" {
+		runtimeOpts.BinaryName = runcBinary
+	}
+	if context.Bool("runc-systemd-cgroup") {
+		if context.String("cgroup") == "" {
+			// runc maps "machine.slice:foo:deadbeef" to "/machine.slice/foo-deadbeef.scope"
+			return nil, errors.New("option --runc-systemd-cgroup requires --cgroup to be set, e.g. \"machine.slice:foo:deadbeef\"")
+		}
+		runtimeOpts.SystemdCgroup = true
+	}
+	if root := context.String("runc-root"); root != "" {
+		runtimeOpts.Root = root
+	}
+
+	return runtimeOpts, nil
+}
+
+func RuntimeOptions(context *cli.Context) (interface{}, error) {
+	// validate first
+	if (context.String("runc-binary") != "" || context.Bool("runc-systemd-cgroup")) &&
+		context.String("runtime") != "io.containerd.runc.v2" {
+		return nil, errors.New("specifying runc-binary and runc-systemd-cgroup is only supported for \"io.containerd.runc.v2\" runtime")
+	}
+
+	if context.String("runtime") == "io.containerd.runc.v2" {
+		return getRuncOptions(context)
+	}
+
+	if configPath := context.String("runtime-config-path"); configPath != "" {
+		return &runtimeoptions.Options{
+			ConfigPath: configPath,
+		}, nil
+	}
+
+	return nil, nil
+}