[8.x] Add relative URL support to ValidateSignature middleware

[BrandonSurowiec]

(This is a resubmission of #30336 by @tonysm.)

I needed to verify a relatively signed route and discovered the signed middleware that I normally reach for didn't support validating relative urls. This is now possible:

Route::get('foo')->middleware('signed:relative');

It helps to avoid writing checks like this in the controller:

public function index(Request $request)
{
    abort_unless($request->hasValidSignature($absolute = false), 403);

    //

I prefer to see what routes are signed in my routes/web.php file, not buried in the depths below.