feat: attach public key

pkg/message/parser/part.go

@@ -1,6 +1,8 @@
 package parser
 
 import (
+	"errors"
+
 	"github.com/emersion/go-message"
 )
 
@@ -10,6 +12,22 @@ type Part struct {
 	children []*Part
 }
 
+func (p *Part) Part(n int) (part *Part, err error) {
+	if len(p.children) < n {
+		return nil, errors.New("no such part")
+	}
+
+	return p.children[n-1], nil
+}
+
+func (p *Part) Parts() (n int) {
+	return len(p.children)
+}
+
+func (p *Part) AddChild(child *Part) {
+	p.children = append(p.children, child)
+}
+
 func (p *Part) visit(w *Walker) (err error) {
 	if err = p.handle(w); err != nil {
 		return