Support multiple OAuth Resource Servers for one user

[Jigsaw5279]

It might be beneficial to support multiple Resource Servers to identify one user. In the current setup the providers would either override each others entry, or create entirely different users for each individual provider. The solution would be to not store the identifier on the user record itself, but on an intermediary table.

This would take some rework, but if this sounds like something the extension should have, I'd happily tackle that