FEATURE: Preload resources via link header (#18475)

Experiment moving from preload tags in the document head to preload information the the response headers.

While this is a minor improvement in most browsers (headers are parsed before the response body), this allows smart proxies like Cloudflare to "learn" from those headers and build HTTP 103 Early Hints for subsequent requests to the same URI, which will allow the user agent to download and parse our JS/CSS while we are waiting for the server to generate and stream the HTML response.

Co-authored-by: Penar Musaraj <pmusaraj@gmail.com>

spec/lib/stylesheet/manager_spec.rb

@@ -148,6 +148,16 @@ RSpec.describe Stylesheet::Manager do
       })
     end
 
+    it "stylesheet_link_tag calls the preload callback when set" do
+      preload_list = []
+      preload_callback = ->(href, type) { preload_list << [href, type] }
+
+      manager = manager(theme.id)
+      expect {
+        manager.stylesheet_link_tag(:desktop_theme, 'all', preload_callback)
+      }.to change(preload_list, :size)
+    end
+
     context "with stylesheet order" do
       let(:z_child_theme) do
         Fabricate(:theme, component: true, name: "ze component").tap do |z|
@@ -638,6 +648,17 @@ RSpec.describe Stylesheet::Manager do
       expect(details1[:new_href]).not_to eq(details2[:new_href])
     end
 
+    it "calls the preload callback when set" do
+      preload_list = []
+      cs = Fabricate(:color_scheme, name: 'Funky')
+      theme = Fabricate(:theme, color_scheme_id: cs.id)
+      preload_callback = ->(href, type) { preload_list << [href, type] }
+
+      expect {
+        manager.color_scheme_stylesheet_link_tag(theme.id, 'all', preload_callback)
+      }.to change(preload_list, :size).by(1)
+    end
+
     context "with theme colors" do
       let(:theme) { Fabricate(:theme).tap { |t|
         t.set_field(target: :common, name: "color_definitions", value: ':root {--special: rebeccapurple;}')