simpleRestProvider: using a custom http header to count items in a collection

[alexisjanvier]

Historically, Simple REST Data Provider uses the http Content-Range header to retrieve the number of items in a collection. But this is a hack of the primary role of this header.

And this can be problematic, for example within an infrastructure using a Varnish that may use, modify or delete this header. We also have feedback indicating that using this header is problematic when you host your application on Versel.

The solution is to use another http header to return the number of collection's items.

This PR introduces a third optional parameter to the simpleRestProvider allowing to define this header.