Support '/path/{parameter}' paths

I noticed that supplying parameters via path is not supported yet, so
here is a simple (and probably naïve) implementation.
Could you do something like this:

   my $self = shift;
   $RES->{id} = $self->param('petId');
   $self->list_pets_get(@_);

And then set ```$RES->{id} = 42``` in the unit test, so we can actually know this method is called?