DEV: Provide API for anonymous cache segments (#8455)

This can be used from a plugin that needs to establish something new in
the anonymous cache. For example `is_ie` for an internet explorer
plugin.

lib/plugin/instance.rb

@@ -82,6 +82,13 @@ class Plugin::Instance
     @idx = 0
   end
 
+  def register_anonymous_cache_key(key, &block)
+    key_method = "key_#{key}"
+    add_to_class(Middleware::AnonymousCache, key_method, &block)
+    Middleware::AnonymousCache.cache_key_segments[key] = key_method
+    Middleware::AnonymousCache.compile_key_builder
+  end
+
   def add_admin_route(label, location)
     @admin_route = { label: label, location: location }
   end