cmd: add new otk-make-grub2-inst-stage external

This external allows us to generate the `org.osbuild.grub2.inst`
for `otk`.

E.g. in PR#193 we can now use the following diff:
```yaml
diff --git a/example/centos/centos-9-x86_64-qcow2.yaml b/example/centos/centos-9-x86_64-qcow2.yaml
index 0d6c0cd..b3693d4 100644
--- a/example/centos/centos-9-x86_64-qcow2.yaml
+++ b/example/centos/centos-9-x86_64-qcow2.yaml
@@ -262,20 +262,9 @@ otk.target.osbuild:
                   ${fs_options.devices}
                 mounts:
                   ${fs_options.mounts}
-              - type: org.osbuild.grub2.inst
-                options:
-                  filename: disk.img
+            - - otk.external.otk-make-grub2-inst-stage:
                   platform: i386-pc
-                  location: 2048
-                  core:
-                    type: mkimage
-                    partlabel: gpt
-                    filesystem: xfs
-                  prefix:
-                    type: partition
-                    partlabel: gpt
-                    number: 2
-                    path: /grub2
+                  filesystem: ${filesystem}
     - name: qcow2
       build: name:build
       stages:
```

cmd/otk-make-grub2-inst-stage/export_test.go

@@ -0,0 +1,3 @@
+package main
+
+var Run = run

cmd/otk-make-grub2-inst-stage/main.go

@@ -0,0 +1,48 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"os"
+
+	"github.com/osbuild/images/pkg/osbuild"
+
+	"github.com/osbuild/images/internal/otkdisk"
+)
+
+type Input struct {
+	Tree Tree `json:"tree"`
+}
+
+type Tree struct {
+	Platform   string       `json:"platform"`
+	Filesystem otkdisk.Data `json:"filesystem"`
+}
+
+func run(r io.Reader, w io.Writer) error {
+	var inp Input
+	if err := json.NewDecoder(r).Decode(&inp); err != nil {
+		return err
+	}
+
+	opts := osbuild.NewGrub2InstStageOption(inp.Tree.Filesystem.Const.Filename, inp.Tree.Filesystem.Const.Internal.PartitionTable, inp.Tree.Platform)
+	stage := osbuild.NewGrub2InstStage(opts)
+
+	out := map[string]interface{}{
+		"tree": stage,
+	}
+	outputJson, err := json.MarshalIndent(out, "", "  ")
+	if err != nil {
+		return fmt.Errorf("cannot marshal response: %w", err)
+	}
+	fmt.Fprintf(w, "%s\n", outputJson)
+	return nil
+}
+
+func main() {
+	if err := run(os.Stdin, os.Stdout); err != nil {
+		fmt.Fprintf(os.Stderr, "error: %v", err.Error())
+		os.Exit(1)
+	}
+}

cmd/otk-make-grub2-inst-stage/main_test.go

@@ -0,0 +1,91 @@
+package main_test
+
+import (
+	"bytes"
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	makeGrub2Inst "github.com/osbuild/images/cmd/otk-make-grub2-inst-stage"
+	"github.com/osbuild/images/internal/common"
+	"github.com/osbuild/images/internal/otkdisk"
+	"github.com/osbuild/images/pkg/disk"
+)
+
+var fakePt = &disk.PartitionTable{
+	Type: "gpt",
+	Partitions: []disk.Partition{
+		{
+			Size:     1 * common.MiB,
+			Start:    1 * common.MiB,
+			Bootable: true,
+			Type:     disk.BIOSBootPartitionGUID,
+			UUID:     disk.BIOSBootPartitionUUID,
+		},
+		{
+			Size: 1 * common.GiB,
+			Payload: &disk.Filesystem{
+				Type:       "ext4",
+				Mountpoint: "/",
+				UUID:       disk.RootPartitionUUID,
+			},
+		},
+	},
+}
+
+// this is not symetrical to the output, this is sad but also
+// okay because the input is really just a dump of the internal
+// disk.PartitionTable so encoding it in json here will not add
+// a benefit for the test
+var minimalInputBase = makeGrub2Inst.Input{
+	Tree: makeGrub2Inst.Tree{
+		Platform: "i386-pc",
+		Filesystem: otkdisk.Data{
+			Const: otkdisk.Const{
+				Internal: otkdisk.Internal{
+					PartitionTable: fakePt,
+				},
+			},
+		},
+	},
+}
+
+var minimalExpectedStages = `{
+  "tree": {
+    "type": "org.osbuild.grub2.inst",
+    "options": {
+      "filename": "disk.img",
+      "platform": "i386-pc",
+      "location": 2048,
+      "core": {
+        "type": "mkimage",
+        "partlabel": "gpt",
+        "filesystem": "ext4"
+      },
+      "prefix": {
+        "type": "partition",
+        "partlabel": "gpt",
+        "number": 1,
+        "path": "/boot/grub2"
+      }
+    }
+  }
+}
+`
+
+func TestIntegration(t *testing.T) {
+	minimalInput := minimalInputBase
+	minimalInput.Tree.Filesystem.Const.Filename = "disk.img"
+	expectedStages := minimalExpectedStages
+
+	inpJSON, err := json.Marshal(&minimalInput)
+	assert.NoError(t, err)
+	fakeStdin := bytes.NewBuffer(inpJSON)
+	fakeStdout := bytes.NewBuffer(nil)
+
+	err = makeGrub2Inst.Run(fakeStdin, fakeStdout)
+	assert.NoError(t, err)
+
+	assert.Equal(t, expectedStages, fakeStdout.String())
+}