feat: provide logout url for openid providers (#1340)

Co-authored-by: kolaente <k@knt.li>
Reviewed-on: https://kolaente.dev/vikunja/api/pulls/1340
Co-authored-by: viehlieb <pf@pragma-shift.net>
Co-committed-by: viehlieb <pf@pragma-shift.net>

pkg/swagger/swagger.yaml

@@ -1120,6 +1120,8 @@ definitions:
         type: string
       key:
         type: string
+      logout_url:
+        type: string
       name:
         type: string
     type: object