[9.x] Add "whereIn" route parameter constraint method

[Propaganistas]

Alternative for #41776, as suggested by @taylorotwell

An enum is often used as a model attribute to simplify DX while still efficiently persisting integers in the database. There are ample of situations where a model's enum would be included in a route as well. Currently Laravel is only able to implicitly resolve string-backed enum route bindings, but that sort of contradicts the aforementioned use case.

By adding a whereIn constraint method, one can easily restrict a parameter to only a few valid values. It's not even limited to enums. Any array will fit.

Route::get('/foo/{bar}')->whereIn('bar', $values);