[api] HTTP scheme support

Consul API providing HTTPClient which resolves DNS names with HTTPS, but can't work with HTTP.
I don't use TLS via internal services but I need to resolve the service name.

Start at the Consul API HTTPClient entry point and trace the HTTPS DNS-resolution path described in the issue. Check how an HTTP service name is handled without TLS, then add coverage for that case; done means HTTP scheme support works for internal services.

Written by the indexing model from the issue text.