feat(dir): add referrer CIDs

Signed-off-by: Bendegúz Csirmaz <csirmazbendeguz@gmail.com>

Author: csirmazbendeguz

api/core/v1/referrer.go

@@ -3,6 +3,11 @@
 
 package v1
 
+import (
+	"encoding/json"
+	"fmt"
+)
+
 // ReferrerObject defines an interface for referrer objects that can be
 // marshaled and unmarshaled to/from RecordReferrer format.
 type ReferrerObject interface {
@@ -18,3 +23,27 @@ type ReferrerObject interface {
 	//   - PublicKey: "agntcy.dir.sign.v1.PublicKey"
 	ReferrerType() string
 }
+
+// Marshal marshals the RecordReferrer similar to how Record.Marshal marshals Record.
+func (r *RecordReferrer) Marshal() ([]byte, error) {
+	if r == nil {
+		return nil, nil
+	}
+
+	jsonBytes, err := json.Marshal(r)
+	if err != nil {
+		return nil, fmt.Errorf("failed to marshal RecordReferrer: %w", err)
+	}
+
+	var normalized any
+	if err := json.Unmarshal(jsonBytes, &normalized); err != nil {
+		return nil, fmt.Errorf("failed to normalize JSON for canonical ordering: %w", err)
+	}
+
+	canonicalBytes, err := json.Marshal(normalized)
+	if err != nil {
+		return nil, fmt.Errorf("failed to marshal normalized JSON with sorted keys: %w", err)
+	}
+
+	return canonicalBytes, nil
+}