Use this ONLY when you need child routes to optionally skip the precondition. For most cases, use PreCondition instead.

Child routes skip with [<SkipAllPreconditions>] or [<SkipPrecondition(typeof<T>)>].

When skipped, the field contains a default value - use _ pattern to ignore it.

Example

 type ItemRoute =
     | List                                  // Inherits parent's OverridablePreCondition
     | [<SkipAllPreconditions>] Public       // Skips it - no auth required

 type Route =
     | Items of OverridablePreCondition<UserId> * ItemRoute

 // In handler, use _ to ignore skipped preconditions:
 match route with
 | Items (_, Public) -> handlePublic ()
 | Items (OverridablePreCondition userId, List) -> handleList userId