[9.x] Allow enum route bindings to have default values

[levu42]

It's possible already to bind route parameters to enums:

enum Status: string {
  case Published = 'published';
  case Draft = 'draft';
}

Route::get('/posts/{status}', function(Status $status) {});

However, it's not yet possible to have default values for optional parameters, as I discovered in #44249:

Route::get('/posts/{status?}', function(Status $status = Status::Published) {});

As discussed in #44249, I made the PR in a way, that it only applies, if the parameter is an enum, so it doesn't interfere with model binding in any way.

I'm not fully happy with the nested ternary operator, making it a bit more complex - I'm happy to change it so some other structure if you think something else is more readable.

I tried to keep the code change minimal to support this while having extensive tests, for them I looked at the existing backed enum binding tests and extended those.

[BrandonSurowiec]

Looks good to me!

[taylorotwell]

Thanks 👍