Suggested improvement for ZNE's get_noisy_circuit function (related to #1305)

[mho291]

Currently, the get_noisy_circuit function aims to create circuits with repeated CNOTs and/or RX gates to increase the noise artificially.

The paper Digital Zero Noise Extrapolation for QEM (https://arxiv.org/abs/2005.10921) has mentioned two other ways of artificially increasing the amount of noise.

The first is called gate folding, where each U_i gate in the circuit is repeated n times U_i --> U_i ( U_i^dagger * U_i )^n

The second is called circuit folding, where the entire circuit (or maybe a sequence of gates) is repeated in the same manner.

I believe this feature will increase the variability and complexity of the generated circuits, allowing for more robust testing and evaluation of quantum error mitigation techniques.

[mho291]

Added circuit folding (global unitary folding) in #1327