feat: initial parser exposing walker/writer

pkg/message/parser/walker.go

@@ -0,0 +1,39 @@
+package parser
+
+type Walker struct {
+	root *Part
+
+	defaultHandler PartHandler
+	typeHandlers   map[string]PartHandler
+	dispHandlers   map[string]DispHandler
+}
+
+type PartHandler func(*Part) error
+type DispHandler func(*Part, PartHandler) error
+
+func newWalker(root *Part) *Walker {
+	return &Walker{
+		root:           root,
+		defaultHandler: func(*Part) (err error) { return },
+		typeHandlers:   make(map[string]PartHandler),
+		dispHandlers:   make(map[string]DispHandler),
+	}
+}
+
+func (w *Walker) Walk() (err error) {
+	return w.root.visit(w)
+}
+
+func (w *Walker) WithDefaultHandler(handler PartHandler) *Walker {
+	w.defaultHandler = handler
+	return w
+}
+func (w *Walker) WithContentTypeHandler(contType string, handler PartHandler) *Walker {
+	w.typeHandlers[contType] = handler
+	return w
+}
+
+func (w *Walker) WithContentDispositionHandler(contDisp string, handler DispHandler) *Walker {
+	w.dispHandlers[contDisp] = handler
+	return w
+}