Generalize URL generation, taking into account protocol #1139
Description
If Cog is behind a proxy, we need generated URLs (e.g., for triggers and service endpoints) to be properly generated.
Currently this is mostly handled by COG_API_URL_HOST et al. We need to also add support for specifying the protocol as well (http vs. https).
In an ideal world, it'd be nice to find a way to be able to optionally take advantage of X-Forwarded-* HTTP headers that can be injected by the proxy. I'm not sure if we can necessarily do that for everything, but it might be a nice feature to have.
Consolidating configuration for our api, trigger, and service endpoints might also be nice; it's kind of a pain to have to specify protocol, host, and port for each of them, when they're likely to be the same (modulo the port) most of the time.