[backport#17261 10/13] Box the wallet: Add multiple keyman maps and loops

Summary:
Add wallet logic for dealing with multiple ScriptPubKeyMan instances. This
doesn't change current behavior because there is still only a single
LegacyScriptPubKeyMan. But in the future the new logic will be used to support
descriptor wallets.

https://github.com/bitcoin/bitcoin/pull/17261/commits/c729afd0a3b74a3943e4c359270beaf3e6ff8a7b

Note: Had to bring in PR18052 to fix an instance of -Wmaybe-uninitialized

Depends on D7858

Partial backport of Core PR17261 and PR18052

Test Plan:

ninja all check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7859