CNOT gate decomposition should be implemented

[csookim]

@sorewachigauyo Paul mentioned that an unroller is needed to decompose the circuit using CNOT, GPI2, RZ, Z, and M. To achieve this, CNOT should be included as a native gate.

Here are the tasks to consider:

• [x] Add CNOT as an attribute of NativeGates.
• [ ] Implement cnot_dec in decompositions.py.
• [ ] Modify the logic of _translate_two_qubit_gates() in unroller.py.

I believe cnot_dec can be implemented based on the existing cz_dec.