[5.7] Add support for multiple classes on contextual binding

[fetzi]

I often need to add contextual binding for different non standard use cases. When doing this I recognized that I often have to define multiple contextual bindings for the same interface.

Consider the following example:

$this->app->when(PhotoController::class)
          ->needs(Filesystem::class)
          ->give(function () {
              return Storage::disk('s3');
          });

$this->app->when(VideoController::class)
          ->needs(Filesystem::class)
          ->give(function () {
              return Storage::disk('s3');
          });

$this->app->when(UploadController::class)
          ->needs(Filesystem::class)
          ->give(function () {
              return Storage::disk('local');
          });

This pull request adds the possibility to define the contextual binding for PhotoController and VideoController at once and should ease and shorten the service container bindings.

$this->app->when([PhotoController::class, VideoController::class])
          ->needs(Filesystem::class)
          ->give(function () {
              return Storage::disk('s3');
          });

$this->app->when(UploadController::class)
          ->needs(Filesystem::class)
          ->give(function () {
              return Storage::disk('local');
          });