FEATURE: Add hooks for email poller plugins (#21384)

While we are unable to support OAUTH2 with pop3 (due to upstream dependency ruby/net-pop#16), we are adding the support for mail pollers plugin. Doing so, it would be possible to write a plugin which then uses other ways (microsoft graph sdk for example) to poll emails from a mailbox.

The idea is that a plugin would define a class which inherits from Email::Poller and defines a poll_mailbox static method which returns an array of strings. Then the plugin could call register_mail_poller(<class_name>) to have it registered. All the configuration (oauth2 tokens, email, etc) could be managed by sitesettings defined in the plugin.

spec/lib/discourse_plugin_registry_spec.rb

@@ -98,6 +98,13 @@ RSpec.describe DiscoursePluginRegistry do
     end
   end
 
+  describe "#mail_pollers" do
+    it "defaults to an empty Set" do
+      registry.reset!
+      expect(registry.mail_pollers).to eq(Set.new)
+    end
+  end
+
   describe ".register_html_builder" do
     it "can register and build html" do
       DiscoursePluginRegistry.register_html_builder(:my_html) { "<b>my html</b>" }