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/plugins/plugins.go

@@ -17,6 +17,7 @@
 package plugins
 
 import (
+	"encoding/json"
 	"fmt"
 	"os"
 	"sort"
@@ -39,6 +40,7 @@ var Command = cli.Command{
 	Usage:   "Provides information about containerd plugins",
 	Subcommands: []cli.Command{
 		listCommand,
+		inspectRuntimeCommand,
 	},
 }
 
@@ -162,3 +164,32 @@ func prettyPlatforms(pspb []*types.Platform) string {
 	sort.Stable(sort.StringSlice(ps))
 	return strings.Join(ps, ",")
 }
+
+var inspectRuntimeCommand = cli.Command{
+	Name:      "inspect-runtime",
+	Usage:     "Display runtime info",
+	ArgsUsage: "[flags]",
+	Flags:     commands.RuntimeFlags,
+	Action: func(context *cli.Context) error {
+		rt := context.String("runtime")
+		rtOptions, err := commands.RuntimeOptions(context)
+		if err != nil {
+			return err
+		}
+		client, ctx, cancel, err := commands.NewClient(context)
+		if err != nil {
+			return err
+		}
+		defer cancel()
+		rtInfo, err := client.RuntimeInfo(ctx, rt, rtOptions)
+		if err != nil {
+			return err
+		}
+		j, err := json.MarshalIndent(rtInfo, "", "    ")
+		if err != nil {
+			return err
+		}
+		_, err = fmt.Fprintln(context.App.Writer, string(j))
+		return err
+	},
+}