REST API: Use `wp.apiRequest` helper in `wp.api` Backbone client

[jnylen0]

Follow-up to #40919. The wp-api.js Backbone client should use wp.apiRequest to do its API calls. This may be accomplished through overriding Backbone.ajax or through a change in the wp-api.js code.

This will allow client-side code to override or modify all WP REST API calls in a single, centralized place.

cc @adamsilverstein for your thoughts on the best way to achieve this.

[adamsilverstein]

@jnylen0 - 41549.diff​ is a first pass at this.

• Override Backbone.ajax with wp.apiRequest.transport - I looked at replacing first with wp.apiRequest, however I don't think we need the additional logic in buildAjaxOptions and this might break something.
• Immediately restore Backbone.ajax after use
• Adjust tests to override wp.apiRequest.transport for testing

Not sure I like this - my concern here is that Backbone authors are used to the idea of being able to override Backbone.ajax to use an alternate method, for example ​this native implementation. This would actually break that (for the client), requiring that they override wp.apiRequest.transport instead.

Punting for now for further consideration.

[jnylen0]

@adamsilverstein I think most of the value in this change is in having wp-api.js use buildAjaxOptions. I could have explained the reasoning better in the ticket, but here is a good example of how this could be helpful: #42807

I am not sure how to handle users overriding Backbone.ajax. It would be good to see an example of this in WordPress context, otherwise I think it is probably not very common.

[adamsilverstein]

@jnylen0 Is this change still needed? If not, let's close this out.

[jnylen0]

Doesn't seem to be needed any more.