osbuildexecutor: introduce osbuildexecutor.Executor interface

Wrap the current osbuildexecutor.Executor in an interface so it's easier
to add different executors, which for instance can run osbuild in a VM.

internal/osbuildexecutor/osbuild-executor.go

@@ -0,0 +1,11 @@
+package osbuildexecutor
+
+import (
+	"io"
+
+	"github.com/osbuild/images/pkg/osbuild"
+)
+
+type Executor interface {
+	RunOSBuild(manifest []byte, store, outputDirectory string, exports, checkpoints, extraEnv []string, result bool, errorWriter io.Writer) (*osbuild.Result, error)
+}