Merge #9332: Let wallet importmulti RPC accept labels for standard scriptPubKeys

Summary:
98ea64cf232c34d4b1aebe738b3956191667cd76 Let wallet importmulti RPC accept labels for standard scriptPubKeys (Russell Yanofsky)

Pull request description:

Allow importmulti RPC to apply address labels when importing standard scriptPubKeys. This makes the importmulti RPC less finnicky about import formats and also simpler internally.

Tree-SHA512: 102426b21239f1fa5f38162dc3f4145572caef76e63906afd786b7aff1670d6cd93456f8d85f737588eedc49c11bef2e1e8019b8b2cbf6097c77b3501b0cab1f

Backport of Core PR9332
I also had to reapply a change from PR14679

Test Plan:

ninja
ninja check
ninja check-functional

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6061