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>

core/runtime/v2/shim/shim.go

@@ -31,6 +31,7 @@ import (
 	"time"
 
 	shimapi "github.com/containerd/containerd/v2/api/runtime/task/v3"
+	"github.com/containerd/containerd/v2/api/types"
 	"github.com/containerd/containerd/v2/pkg/events"
 	"github.com/containerd/containerd/v2/pkg/namespaces"
 	"github.com/containerd/containerd/v2/pkg/shutdown"
@@ -79,6 +80,7 @@ type Manager interface {
 	Name() string
 	Start(ctx context.Context, id string, opts StartOpts) (BootstrapParams, error)
 	Stop(ctx context.Context, id string) (StopStatus, error)
+	Info(ctx context.Context, optionsR io.Reader) (*types.RuntimeInfo, error)
 }
 
 // OptsKey is the context key for the Opts value.
@@ -116,6 +118,7 @@ type TTRPCServerOptioner interface {
 var (
 	debugFlag            bool
 	versionFlag          bool
+	infoFlag             bool
 	id                   string
 	namespaceFlag        string
 	socketFlag           string
@@ -135,6 +138,9 @@ const (
 func parseFlags() {
 	flag.BoolVar(&debugFlag, "debug", false, "enable debug output in logs")
 	flag.BoolVar(&versionFlag, "v", false, "show the shim version and exit")
+	// "info" is not a subcommand, because old shims produce very confusing errors for unknown subcommands
+	// https://github.com/containerd/containerd/pull/8509#discussion_r1210021403
+	flag.BoolVar(&infoFlag, "info", false, "get the option protobuf from stdin, print the shim info protobuf to stdout, and exit")
 	flag.StringVar(&namespaceFlag, "namespace", "", "namespace that owns the shim")
 	flag.StringVar(&id, "id", "", "id of the task")
 	flag.StringVar(&socketFlag, "socket", "", "socket path to serve")
@@ -195,6 +201,19 @@ func Run(ctx context.Context, manager Manager, opts ...BinaryOpts) {
 	}
 }
 
+func runInfo(ctx context.Context, manager Manager) error {
+	info, err := manager.Info(ctx, os.Stdin)
+	if err != nil {
+		return err
+	}
+	infoB, err := proto.Marshal(info)
+	if err != nil {
+		return err
+	}
+	_, err = os.Stdout.Write(infoB)
+	return err
+}
+
 func run(ctx context.Context, manager Manager, config Config) error {
 	parseFlags()
 	if versionFlag {
@@ -206,6 +225,10 @@ func run(ctx context.Context, manager Manager, config Config) error {
 		return nil
 	}
 
+	if infoFlag {
+		return runInfo(ctx, manager)
+	}
+
 	if namespaceFlag == "" {
 		return fmt.Errorf("shim namespace cannot be empty")
 	}