[feature request] Swappable sign/verify functions?

For embedded systems with hardware accelerated ECSDA engines, it would be useful to be able to expose swappable function pointers for signing and verifying.

**I have a fork which does this [here](https://github.com/ElementsProject/libwally-core/compare/master...jmecom:libwally-core:master).**

It's a little bit weird, because the `libsecp256k1` context is still initialized unconditionally. This is an okay tradeoff, IMO, because hardware accelerated Schnorr signing is extremely rare, so in practice a caller would likely still want `libsecp256k1` for Schnorr, even if they are accelerating ECDSA.

Is this something that makes sense for the project to support? If so, I'm happy to clean up and test my fork and PR it.