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

spec/services/activitypub/process_collection_service_spec.rb

@@ -1,9 +1,10 @@
 require 'rails_helper'
 
 RSpec.describe ActivityPub::ProcessCollectionService do
-  subject { ActivityPub::ProcessCollectionService.new }
+  subject { described_class.new }
 
   describe '#call' do
-    pending
+    context 'when actor is the sender'
+    context 'when actor differs from sender'
   end
 end