Add support for inferring descriptors from scripts

Summary:
This is a partial backport of Core PR14477 : https://github.com/bitcoin/bitcoin/pull/14477/commits/4d78bd93b5bdf886e743022e80f4edb8a982cf0d

I made InferPubkey static because it is only called from there.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6124