Add json codec (#24)

Block Encode/Decode is really nice and id would be nice to use it with
JSON :)

core/ipld/codec/json/json.go

@@ -0,0 +1,34 @@
+package json
+
+import (
+	"github.com/ipld/go-ipld-prime"
+	"github.com/ipld/go-ipld-prime/codec/dagjson"
+	"github.com/ipld/go-ipld-prime/schema"
+)
+
+const Code = 0x0129
+
+type codec struct{}
+
+func (codec) Code() uint64 {
+	return Code
+}
+
+func (codec) Encode(val any, typ schema.Type) ([]byte, error) {
+	return Encode(val, typ)
+}
+
+func (codec) Decode(b []byte, bind any, typ schema.Type) error {
+	return Decode(b, bind, typ)
+}
+
+var Codec = codec{}
+
+func Encode(val any, typ schema.Type) ([]byte, error) {
+	return ipld.Marshal(dagjson.Encode, val, typ)
+}
+
+func Decode(b []byte, bind any, typ schema.Type) error {
+	_, err := ipld.Unmarshal(b, dagjson.Decode, bind, typ)
+	return err
+}