Introduce WalletDescriptor class

Summary:
WalletDescriptor is a Descriptor with other wallet metadata

Backport of Core PR16528 part [6/43] : https://github.com/bitcoin/bitcoin/pull/16528/commits/3194a7f88ac1a32997b390b4f188c4b6a4af04a5

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8404