change instance of manager

[uesley]

it allows laravel service container to use mocked class instead concrete manager

Changes In Code

FeatureGuard is the class that resolve when middleware like feature:my-feature is declared in routes

the parameter $manager in the construct method is for a concrete class. Substituting that for the YlsIdeas\FeatureFlags\Contracts\Features interface allows the service container of laravel to replace with a fake instance when use the fake of facade Features::fake(['my-feature' => true]).

Issue ticket number / Business Case

#70

Checklist before requesting a review

• I have written PHPUnit tests.
• I have updated the documentation and opened a pull request within
  the feature flags documentation repo.
• I have checked code styles, PHPStan etc. pass.
• I have provided an issue/business case.
• I have added the enhancement label for a new feature.