Optionally force HTTP while harvesting

[damb]

Due to https://github.com/EIDA/etc/issues/36 routes are configured via HTTPS, too. Though, eidaws-federator's backend is not prepared to fully rely on HTTPS. Since all EIDA endpoint nodes still provide HTTP and eidaws-federator's backend does not strictly require HTTPS to be used as a simple workaround HTTPS routing URLs may be (optionally) overridden with the corresponding HTTP routing URL while harvesting. This solution should work out as long as:

• EIDA endpoint nodes provide HTTP APIs
• eidaws-federator does not implement restricted data access.