EZP-23277 REST siteaccess can be set via X-Siteaccess header

Without this change, the classical siteacces matching rules applied, but the request URI wasn't changed if uri matching was used. As a consequence, uri based matching was unusable (and the rewrite rules didn't allow it by default).

This unifies how the REST API v1 behaves as compared to v2, by allowing the SA to be specified via a custom header, such as `X-Siteaccess: site_admin`.

If the header is not passed, matching will be tried as it was before.