**Backport:** https://codeberg.org/forgejo/forgejo/pulls/12202
Currently authentication methods return information in two forms: they return who was authenticated as a `*user_model.User`, and then they insert key-values into `ctx.Data` which has critical impact on how the authenticated request is treated. This PR changes the authentication methods to return structured data in the form of an `AuthenticationResult`, with all the key-value information in `ctx.Data` being moved into methods on the `AuthenticationResult` interface.
Authentication workflows in Forgejo are a real mess. This is the first step in trying to clean it up and make the code predictable and reasonable, and is both follow-up work that was identified from the repo-specific access tokens (where the `"ApiTokenReducer"` key-value was added), and is pre-requisite work to future JWT enhancements that are [being discussed](https://codeberg.org/forgejo/forgejo/issues/3571#issuecomment-13268004).
Co-authored-by: Mathieu Fenniak <mathieu@fenniak.net>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/12462
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
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