Add public key declaration to actors (#4215)

app/serializers/activitypub/public_key_serializer.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+class ActivityPub::PublicKeySerializer < ActiveModel::Serializer
+  attributes :id, :owner, :public_key_pem
+
+  def id
+    [ActivityPub::TagManager.instance.uri_for(object), '#main-key'].join
+  end
+
+  def owner
+    ActivityPub::TagManager.instance.uri_for(object)
+  end
+
+  def public_key_pem
+    object.public_key
+  end
+end