Add scheme to TraefikRouteProvider

[sed-i]

Problem

Grafana, which uses traefik_route, is currently unable to tell if the ingress endpoint is http or https.

Solution

This PR adds a scheme arg, so that traefik could inform over relation data which scheme it is using.

In tandem with:

• https://github.com/canonical/traefik-k8s-operator/pull/234
• https://github.com/canonical/grafana-k8s-operator/pull/257