Identityserver4.admin: Adding public origin to configuration of STS

[xmichaelx]

In many cases STS is behind some proxy or load balancing so the default origin used in discovery document for prepending endpoints inferred from request may be a local one inferred from passed request.

PublicOrigin is an option that can be set to mitigate that, we should add it to the configuration of STS.

[aiscrim]

I am developing a PR to deal with this and other issues related to load balancing configurations. Stay tuned! :)

[skoruba]

Thanks @aiscrim

[skoruba]

Done in master. Thx