Extend the API

[grevian]

Update the specification and regenerate it in the repository

The service now fails to build because our implementation no longer matches the specification

# github.com/grevian/petstore-demo/service
service\pet.go:11:32: cannot use (*petAPIService)(nil) (value of type *petAPIService) as openapi.PetApiServicer value in variable declaration: *petAPIService does not implement openapi.PetApiServicer (missing method GetPetStatusById)

Which can be resolved by adding an implementation of GetPetStatusById to the Pet service implementation

func (p petAPIService) GetPetStatusById(ctx context.Context, petId int64) (openapi.ImplResponse, error) {
	pet, err := p.storage.LoadPet(petId)
	if err != nil {
		return openapi.Response(http.StatusNotFound, nil), err
	}
	return openapi.Response(http.StatusOK, pet.Status), nil
}

See #7 for the breaking API change that leads to this required implementation change