Implement SignMessage for descriptor wallets
Implement SignMessage for descriptor wallets

Summary: Backport of Core [[https://github.com/bitcoin/bitcoin/pull/16528 | 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

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