[8.x] Allowing skipping TransformRequests middleware via Closure

[taylorotwell]

Currently, Livewire has to do some nasty hacking to skip these middleware on certain requests and the approach is not suitable for use with Octane. Adding this feature will allow Livewire and other libraries to register a callback with this base middleware during the framework's boot process to determine if the middleware should be skipped. That callback will receive the current request on each invocation.

TrimStrings::skipWhen(fn ($request) => shouldBeSkipped($request));
ConvertEmptyStringsToNull::skipWhen(fn ($request) => shouldBeSkipped($request));