pipeline/target: implement as variant types

Go doesn't really do variants, so we must somehow emulate it. The
json objects we use are essentially tagged unions, with a `name`
field in reverse domain name notation identifying the type and a
type specific 'options' object.

In Go we represent this by having an BarOptions interface, which
implements a private method `isBarOptions()`, making sure that only
types in the same package are able to implement it. Each type FooBar
that should belong to the variant implements the interface, and a
constructor `NewFooBar(options *FooBarOptions) *Bar` that makes sure
the `name` field is set correctly.

This would be enough to represent our types and marshal them into
JSON, but unmarshalling would not work (json does not know about
our tags, so would not know what concrete types to demarshal to).
We therefore must also implement the Unmarshall interface for Bar,
to select the right types for the Options field.

We implement his logic for Target, Stage and Assembler. A handful
of concrete types are also implemented, matching what osbuild
supports.

Signed-off-by: Tom Gundersen <teg@jklm.no>

internal/pipeline/tar_assembler.go

@@ -0,0 +1,25 @@
+package pipeline
+
+type TarAssemblerOptions struct {
+	Filename    string `json:"filename"`
+	Compression string `json:"compression,omitempty"`
+}
+
+func (TarAssemblerOptions) isAssemblerOptions() {}
+
+func NewTarAssemblerOptions(filename string) *TarAssemblerOptions {
+	return &TarAssemblerOptions{
+		Filename: filename,
+	}
+}
+
+func NewTarAssembler(options *TarAssemblerOptions) *Assembler {
+	return &Assembler{
+		Name:    "org.osbuild.tar",
+		Options: options,
+	}
+}
+
+func (options *TarAssemblerOptions) SetCompression(compression string) {
+	options.Compression = compression
+}