Bitcoin ABC Schnorr signature support for Nexa

[Dolaned]

Description

Whilst looking over the code in cx_schnorr.c i noticed that the schnorr version used Bitcoin ABC is not implemented.

Nexa nexa.org uses this schnorr implememtation, is it possible to get this implemented or open a MR for it?

i have completed the remainder of the implementation for Nexa, just need clarification on this

Thank you !