Add support for X-Forwarded-Proto

When running multiple Trellis nodes behind a TLS-terminating load balancer, it would be nice if the in-domain triples to used the `https` scheme. This can currently be achieved by setting the `trellis.http.base-url` property (or `TRELLIS_HTTP_BASE_URL` env variable), but supporting the `X-Forwarded-Proto` header would add support for this independent of setting a server-wide base URL.