Add handling of Linked Data Signatures in payloads (#4687)

* Add handling of Linked Data Signatures in payloads

* Add a way to sign JSON, fix canonicalization of signature options

* Fix signatureValue encoding, send out signed JSON when distributing

* Add missing security context

app/services/reblog_service.rb

@@ -42,10 +42,10 @@ class ReblogService < BaseService
   end
 
   def build_json(reblog)
-    ActiveModelSerializers::SerializableResource.new(
+    Oj.dump(ActivityPub::LinkedDataSignature.new(ActiveModelSerializers::SerializableResource.new(
       reblog,
       serializer: ActivityPub::ActivitySerializer,
       adapter: ActivityPub::Adapter
-    ).to_json
+    ).as_json).sign!(reblog.account))
   end
 end