HTTP signatures (#4146)

* Add Request class with HTTP signature generator

Spec: https://tools.ietf.org/html/draft-cavage-http-signatures-06

* Add HTTP signature verification concern

* Add test for SignatureVerification concern

* Add basic test for Request class

* Make PuSH subscribe/unsubscribe requests use new Request class

Accidentally fix lease_seconds not being set and sent properly, and
change the new minimum subscription duration to 1 day

* Make all PuSH workers use new Request class

* Make Salmon sender use new Request class

* Make FetchLinkService use new Request class

* Make FetchAtomService use the new Request class

* Make Remotable use the new Request class

* Make ResolveRemoteAccountService use the new Request class

* Add more tests

* Allow +-30 seconds window for signed request to remain valid

* Disable time window validation for signed requests, restore 7 days
as PuSH subscription duration (which was previous default due to a bug)

app/services/fetch_atom_service.rb

@@ -1,16 +1,14 @@
 # frozen_string_literal: true
 
 class FetchAtomService < BaseService
-  include HttpHelper
-
   def call(url)
     return if url.blank?
 
-    response = http_client.head(url)
+    response = Request.new(:head, url).perform
 
     Rails.logger.debug "Remote status HEAD request returned code #{response.code}"
 
-    response = http_client.get(url) if response.code == 405
+    response = Request.new(:get, url).perform if response.code == 405
 
     Rails.logger.debug "Remote status GET request returned code #{response.code}"
 
@@ -49,6 +47,6 @@ class FetchAtomService < BaseService
   end
 
   def fetch(url)
-    http_client.get(url).to_s
+    Request.new(:get, url).perform.to_s
   end
 end