Function origin_of 

fn origin_of(client_id: &str) -> String

The scheme, host and port of a client_id.

plan/app-allowlist.md: the origin is the only part of a client_id that is a fact about the world, since serving a metadata document there means controlling that host. Falls back to the whole string when it will not parse as a URL, which client_metadata::resolve_client has already ruled out by the time this is called.