Add magnitude approximation protocol

[NoureldinYosri]

• adds the magnitude approximation protocol to approximate general unitaries
• fixes the docstring for the Z-rotation protocols to be $e^{i\theta Z}$ ( $= Rz(-2\theta)$ ) instead of $Rz(2\theta)$

[NoureldinYosri]

@mpharrigan can you take a look at this one as well?

[mpharrigan]

is it possible to update the convention in a future PR to be exp[-i theta/2 z]

[mpharrigan]

lgtm 👍

[NoureldinYosri]

is it possible to update the convention in a future PR to be exp[-i theta/2 z]

at the moment it matches the convention from arXiv:2203.10064, personally I don't have a preference to either, so yes it can be changed