Add multisig descriptor templates

[andreasgriffin]

Describe the enhancement

There are single-sig Descriptors templates, like:

bdk.Descriptor.new_bip86_public
bdk.Descriptor.new_bip86

It would be a big improvement if there are also multisig templates for p2sh-p2wsh and p2wsh

Use case

It makes it safer to create a multisig descriptor.

[thunderbiscuit]

This is basically exposing what would be made available by bitcoindevkit/bdk_wallet#145.