This adds back the `ajax.transport` option, which allows anyone to
use their own AJAX backend. By default, `jQuery.ajax` is used as the
default transport backend.
**Breaking change:** Instead of taking `params` that contains a
`success` and `error` keys for the callback, two new callback
parameters are passed alongside of `params` that should be called
when the request succeeds or fails.
This closes https://github.com/select2/select2/issues/2987.
4c563a3d06