Ensure support for endpoint validators running on `provides` side

Currently endpoint validators are only run from the "requiring" side of the relationship. They should run for `provides` as well.

An example is `prometheus_scrape`, where the requirer provides a metrics http endpiont. There isn't anything for the requirer to do, instead the provider should check that the metrics endpoint is reachable.

Start by tracing where endpoint validators are invoked from the requiring side, then inspect the prometheus_scrape relationship example described in the issue. Done means validators also run from the provides side so the provider can check that the metrics endpoint is reachable.

Written by the indexing model from the issue text.