Support Stackspin OIDC federation
Longer-term, we'd like to make it possible to have Stackspin instances federate, so you can use your account on one Stackspin instance to log in to another (if that has been set up for that pair), and get some level of access.
As a first step, I want to try to have basic OIDC federation working. The immediate use-case is our Harbor registry: that's a separate Stackspin instance, and I'd like to use our stackspin.net accounts to log in to it.
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