[wallet] Allow descriptor imports with importmulti

Summary:
This is a partial backport of Core PR14491 : https://github.com/bitcoin/bitcoin/pull/14491/commits/9f48053d8f9a1feacc96d7e2a00c8a3a67576948

Depends on D6190

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6191