osbuild: Add "labels" field into the SELinux stage

The SELinux stage supports labeling specific files with specific labels
like this:
7b0db90c76/stages/org.osbuild.selinux (L37)

This can be useful for preventing some SELinux issues during the image
build. You can find example usage here:
7b0db90c76/test/data/stages/selinux/test_basic.json (L5)

And more reasoning here:
e80130a830

internal/osbuild/selinux_stage.go

@@ -5,7 +5,8 @@ package osbuild
 // A file contexts configuration file is sepcified that describes
 // the filesystem labels to apply to the image.
 type SELinuxStageOptions struct {
-	FileContexts string `json:"file_contexts"`
+	FileContexts string            `json:"file_contexts"`
+	Labels       map[string]string `json:"labels,omitempty"`
 }
 
 func (SELinuxStageOptions) isStageOptions() {}