[Engines, Backends] Add JAX Engine and Horqrux Backend

[dominikandreasseitz]

Addresses #93

1. This MR adds an Engine to qadence which is uses to instantiate a DifferentiableBackend which can be either JAX or torch.
2. Each backend gets a engine field which is uses by backend_factory to import either a torch or jax differentiable backend.
3. Add a new backend called 'horqrux' https://github.com/pasqal-io/horqrux, a differntiable state vector simulator in JAX
4. Add diff_modes GPSR and AD to horqrux backend
5. Add low-level tutorial to train qadence circuits in JAX

[GJBoth]

LGTM, thanks so much for this :-)