Add additional information to `Policy` structure

[notmandatory]

As discussed in #594 if we can somehow add the below information to the Policy structure it would be more useful in displaying the general signing conditions for a Descriptor to users.

• signing xkey checksums
• signing xkey origin (master key fingerprint and derivation path), if available
• script type (ie. sh(), wsh(), sh(wsh), tr(), etc.)

[bucko13]

I'm not sure this necessarily has to play a role, but thought it was at least worth raising and having for the record this recent discussion from the ledger team on the developer mailing list about a "wallet policy" extension for descriptors:

https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-May/020423.html

[notmandatory]

The plan going forward for bdk is to replace all the policy stuff with the rust_miniscript planning module approach. See: bitcoindevkit/bdk_wallet#4, bitcoindevkit/bdk_wallet#64, and bitcoindevkit/bdk_wallet#164.