Change type of the 'logger' attribute to MaybeConsumerOf

When building the serializer engine, App.pm uses a conditional, based on
which it passes or not a logger as a constructor parameter to the
serializer engine. Because of this, logger can be undef, so the type
sould be MaybeConsumerOf.
Fix important typo

Dancer2::Core::Logger does not exists in the Dancer2 namespace. It
should be Dancer2::Core::Role::Logger