Basically, add a new option in the New World menu to spawn a donkey alongside you when you first enter an world.
OPTIONAL: For dedicated server, add a new option in server.properties (or in <modname>-server.toml if not possible) to spawn a donkey the first time every player joins the server.
OPTIONAL 2: Maybe add a toggleable option to choose other pack animal (horse, llama, camel), but the main focus of this request are donkeys 🦾🫏