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/lib/provider_discovery.rb

@@ -1,11 +1,9 @@
 # frozen_string_literal: true
 
 class ProviderDiscovery < OEmbed::ProviderDiscovery
-  extend HttpHelper
-
   class << self
     def discover_provider(url, options = {})
-      res    = http_client.get(url)
+      res    = Request.new(:get, url).perform
       format = options[:format]
 
       raise OEmbed::NotFound, url if res.code != 200 || res.mime_type != 'text/html'