authz/github: validate provider against default github URL if not set
Created by: bobheadxi
I assumed a URL would be set, and only tested with set URL. In dogfood though am seeing the warning, since the github provider there doesn't have an explicit URL. If one is set, the error goes away.
This change moves the default GitHub URL to package schema
and also makes the warning message a bit clearer regarding what it is referring to. Also cleans up some of the utilities there.
Follow-up to https://github.com/sourcegraph/sourcegraph/pull/24526