support protocol relative service url

at the moment config.yml requires the scheme of the internal service to be specified,
this prevents internal services, like reverse proxies, to determine which protocol is used and behave accordingly;
can http and https be supported for a single service? e.g. a protocol-relative URL:

```
tunnel: ***
credentials-file: /home/nonroot/.cloudflared/***.json

ingress:
- hostname: "*.example.com"
service: //nginx-proxy-manager

- service: http_status:404
```