Add stable marshal of meta & verify headers

It uses unstable implementation for now.

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

service/v2/marshal.go

@@ -0,0 +1,21 @@
+package v2
+
+func (m *RequestMetaHeader) StableMarshal(buf []byte) ([]byte, error) {
+	// fixme: implement stable
+	return m.Marshal()
+}
+
+func (m *RequestMetaHeader) StableSize() int {
+	// fixme: implement stable
+	return m.Size()
+}
+
+func (m *RequestVerificationHeader) StableMarshal(buf []byte) ([]byte, error) {
+	// fixme: implement stable
+	return m.Marshal()
+}
+
+func (m *RequestVerificationHeader) StableSize() int {
+	// fixme: implement stable
+	return m.Size()
+}