cli: add multiwallet capability to GetNewAddress and -generate

Summary:
This is a backport of core#19133 [8/10]
https://github.com/bitcoin/bitcoin/pull/19133/commits/4b859cfff9965eb07044f4d104398cb0e7ab127e

Depends on D9947

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9948