Add public key declaration to actors (#4215)

app/serializers/activitypub/actor_serializer.rb

@@ -7,6 +7,8 @@ class ActivityPub::ActorSerializer < ActiveModel::Serializer
              :inbox, :outbox, :preferred_username,
              :name, :summary, :icon, :image
 
+  has_one :public_key, serializer: ActivityPub::PublicKeySerializer
+
   def id
     account_url(object)
   end
@@ -50,4 +52,8 @@ class ActivityPub::ActorSerializer < ActiveModel::Serializer
   def image
     full_asset_url(object.header.url(:original))
   end
+
+  def public_key
+    object
+  end
 end

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