osbuild: add squashfs stage

Add support for the org.osbuild.squashfs stage.

internal/osbuild/squashfs_stage.go

@@ -0,0 +1,17 @@
+package osbuild
+
+type SquashfsStageOptions struct {
+	Filename string `json:"filename"`
+
+	Compression FSCompression `json:"compression"`
+}
+
+func (SquashfsStageOptions) isStageOptions() {}
+
+func NewSquashfsStage(options *SquashfsStageOptions, inputPipeline string) *Stage {
+	return &Stage{
+		Type:    "org.osbuild.squashfs",
+		Options: options,
+		Inputs:  NewPipelineTreeInputs("tree", inputPipeline),
+	}
+}