Migrate QGANs to new interfaces

[stefan-woerner]

Migrate the currently existing QGAN implementation to the new NN interfaces in Qiskit ML:

• [ ] Introduce classical NN that implements NN interface
• [ ] Add "1 step" to optimizers (where possible, otherwise throw exception)
• [ ] Introduce QGAN that takes NN for generator (sampling) and discriminator (requires design first)
• [ ] Introduce required loss function (can we get gradients here without getting eval for all x?)