Consider exposing supported DG service providers in headers

[phoglenix]

Feedback: On an initial cold request, when server-side rendering a page, it would be useful to know whether the DGAPI is available, so you can know whether to render a buy button etc.
Currently it is only possible to know by running JS after the page is rendered.

We should consider adding headers to expose information on whether the DGAPI will be available (and with which service providers).