Set X-Requested-With header on Openverse.org requests

[sarayourfriend]

Problem

It currently requires two separate headers to determine where requests are coming from. We can use User agent, but it is flaky and cannot be used on the frontend (browsers disallow it). We can also use the application name since #3369, but that doesn't work for the frontend because it doesn't make requests using the client id.

Cloudflare supports the X-Requested-With header especially, and it would make it possible for us to easily disambiguate where requests are coming from.

Description

Set the X-Requested-With header on requests sent from Openverse.org to something sensible to identify requests coming from the frontend. Use a suffix to disambiguate SSR from client side.

We can also ask for a similar thing to get implemented in Gutenberg's Openverse media integration and in Jetpack.