Add subresource integrity for JS and CSS assets (#15096)

Fix #2744

lib/webpacker/manifest_extensions.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+module Webpacker::ManifestExtensions
+  def lookup(name, pack_type = {})
+    asset = super
+
+    if pack_type[:with_integrity] && asset.respond_to?(:dig)
+      [asset.dig('src'), asset.dig('integrity')]
+    elsif asset.respond_to?(:dig)
+      asset.dig('src')
+    else
+      asset
+    end
+  end
+end
+
+Webpacker::Manifest.prepend(Webpacker::ManifestExtensions)