feat: add dag-json codec types

Author: alanshaw

ipld/codec/dagjson/dagjson.go

@@ -0,0 +1,15 @@
+package dagjson
+
+import (
+	jsg "github.com/alanshaw/dag-json-gen"
+)
+
+const Code = 0x0129
+
+type DagJsonMarshaler = jsg.DagJsonMarshaler
+type DagJsonUnmarshaler = jsg.DagJsonUnmarshaler
+
+type DagJsonMarshalable interface {
+	DagJsonMarshaler
+	DagJsonUnmarshaler
+}