Wallet Policies for Descriptor Wallets.

Defines wallet policies, a compact form of descriptors that hardware signing devices can store and show to users to check.

Hardware signing devices have little memory and small screens, yet users should be able to check which account a payment uses. Wallet policies trim descriptors to the essentials and list the extended public keys separately, so a whole account, with its receive and change addresses, can be registered and reviewed on the device.