[backport#16237 1/3] Add GetNewDestination to CWallet to fetch new destinations

Summary:
Instead of having the same multiple lines of code everywhere
that new destinations are fetched, introduce GetNewDestination as
a member function of CWallet which does the key fetching, label
setting, script generation, and destination generation.

https://github.com/bitcoin/bitcoin/pull/16237/commits/172213be5b174243dc501c1103ad5fe2fee67a16

Depends on D6714

Partial backport of Core PR16237

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, jasonbcox, deadalnix

Reviewed By: #bitcoin_abc, jasonbcox, deadalnix

Subscribers: deadalnix, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6716