Add a MenuManager service

As a follow-up to #1984702: Convert menu.module's page callbacks to Controllers → , we should introduce a MenuManager service to encapsulate or replace many of the helper functions left in menu.module

See #1951268: Convert /forum and /forum/% to new router, remove forum_forum_load(), forum_get_topics(), create Forum service → for an example of a manager service
See _menu_get_options() for an example of a function we should replace