Ajax: Support an alternative completeCallback API for transports

[mgol]

Summary

Apart from the existing API:

function( status, statusText, responses, headers ) {}

a new API is now available:

function( { status, statusText, responses, headers } ) {}

This makes it possible to add new parameters in the future without relying on
their order among parameters and being able to provide them selectively.

Ref gh-4405

+37 bytes. Not changing existing transports would make that smaller but we'll need to change the XHR one anyway to land #4405 and then the build gets smaller if we update it in all the places I modified in the PR.

Note: This still needs tests. That said, we don't have any direct tests for jQuery.ajaxTransport, its only implicitly tested by the virtue of two core transports using this API. Therefore, before landing this I'd like to write a few tests for the API first. I'm opening this draft PR now to gain feedback whether this is a direction in which we'd like to go.

Checklist

• All authors have signed the CLA at https://cla.js.foundation/jquery/jquery
• New tests have been added to show the fix or feature works
• Grunt build and unit tests pass locally with these changes
• If needed, a docs issue/PR was created at https://github.com/jquery/api.jquery.com

[mgol]

Closing & re-opening the PR to trigger the EasyCLA check...