Support for legacy and p2sh-witness_v0 descriptors

[SomberNight]

From README:

This library provides an implementation of Segwit-native Output Descriptors and of Miniscript (to be used within wsh() descriptors), with a minimal amount of dependencies.

Indeed I see descriptors/__init__.py is missing sh-nested witness_v0 and legacy descriptors (e.g. pkh(), bare-multi(), etc). I don't mean ~miniscript support inside sh(), just e.g. sh(multi()) or sh(wpk()) that Bitcoin Core and some other projects support. Are they out of scope or just haven't been implemented yet?

[darosior]

I'm on the fence about it. I feel like those should not be the primary focus of this library. But i can see how it would be handy to have some support of legacy stuff for potential downstream users that support them. Let's say that: if it only comes at the cost of code maintenance (for instance it doesn't introduce new dependencies), i'd be happy to merge a patchset adding support for those descriptors.