Restore the .oidcCallback route for external authentication. (#5391)

* Restore the .oidcCallback route (partially reverts #3461) for external authentication.

* Make sure OIDC also works for non-http URLs.

* Remove oidcAuthentication from the state machine.

There isn't a reliable way to detect failure/cancellation when e.g. the user returns from an external app without interacting with the MAS page.

ElementX/Sources/Application/Settings/AppSettings.swift

@@ -251,8 +251,9 @@ final class AppSettings {
     
     /// Any pre-defined static client registrations for OIDC issuers.
     let oidcStaticRegistrations: [URL: String] = ["https://id.thirdroom.io/realms/thirdroom": "elementx"]
-    /// The redirect URL used for OIDC. This no longer uses universal links so we don't need the bundle ID to avoid conflicts between Element X, Nightly and PR builds.
-    private(set) var oidcRedirectURL: URL = "https://element.io/oidc/login"
+    /// The redirect URL used for OIDC. For the normal case we don't actually need the bundle ID as the web authentication session handles the redirect internally.
+    /// However in the case where MAS sends the user to an external app, we need to make sure that the system will open the correct variant of the app (e.g. Nightly).
+    private(set) var oidcRedirectURL: URL! = URL(string: "https://element.io/oauth/ios/\(InfoPlistReader.main.bundleIdentifier)")
     
     private(set) lazy var oidcConfiguration = OIDCConfiguration(clientName: InfoPlistReader.main.bundleDisplayName,
                                                                 redirectURI: oidcRedirectURL,