[Enhance] Add 'backend' argument to product_state

[dominikandreasseitz]

Description:

Refactor [product_state]((https://github.com/pasqal-io/qadence/blob/main/qadence/states.py#L187), such that it accepts a backend: str argument and returns a product state in the backends native representation. A native representation uses native types for the backend in question.

Implementation:

1. This can for example be achieved by simply using product_block and calling qadence.run on it using the particular requested backend.

2. Add tests which ensure that your implementation works for the backends pyqtorch and horqrux