Enhancement: API to create a session shared user

[uplift]

I have a use case where we need to create the user in the forum database when they login to our website so we can have a forum uid for API requests. I thought about just using the create user API (write-api plugin) but then I'd lose the session-sharing lookup ability to check the forum ID based on their website ID.

Would it be possible to add an API that basically takes the request body instead of the cookie body and run that through a modified version of the process function? I'll send a rough PR with something I think should work.