Add support for choosing basis in `InitAncilla` and `PostSelect`

[anurudhp]

Hi! I am currently implementing a lowering from Qualtran to pyzx (quantumlib/Qualtran#1550).

I'm producing a zx.Circuit, and as qualtran natively supports states and effects, it would be very useful to have atomic zx gates that can represent these. I see that InitAncilla and PostSelect are there, but they add a Z spider, i.e. $\ket+$ and $\bra+$ respectively.

Could they be extended with flags to select the basis and basis state (i.e. one of $\ket0, \ket1, \ket+, \ket-$)? Or is there a more direct way to add states and effects?