Auth server is not uniquely identifiable

[pg9182]

We've had multiple people having issues with authentication caused by misconfiguring the server to have overlapping auth ports, which causes only one of them to get the player auth data and return odd authentication errors. This is mostly an issue when port forwarding or using Docker.

To make it easier to troubleshoot these cases, it should be possible to uniquely identify game servers for each auth port.

To solve this with the current API while maintaining backwards compatibility, I propose adding a server ID header to the auth server responses, which will be validated by Atlas if present.