[backport#16227 8/8] Move WatchOnly stuff from SigningProvider to CWallet

Summary:
https://github.com/bitcoin/bitcoin/pull/16227/commits/93ce4a0b6fb54efb1f424a71dfc09cc33307e5b9?file-filters%5B%5D=.h#diff-12635a58447c65585f51d32b7e04075bR1051

Depends on D6560

Concludes backport of Core PR16227

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, nakihito

Reviewed By: nakihito

Subscribers: nakihito

Differential Revision: https://reviews.bitcoinabc.org/D6561