Implement SignMessage for descriptor wallets

Summary: Backport of Core PR16528 [29/43] : https://github.com/bitcoin/bitcoin/pull/16528/commits/84b4978c02102171775c77a45f6ec198930f0a88

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8448