Fix: importmulti only imports origin info for PKH outputs

Summary:

• Keep full pubkeys in FlatSigningProvider::origins
• Import all origin info in importmulti; even for non-importing pubkeys
• Take non-importing keys into account for spendability warning in descriptor import

This is a backport of Core PR15749

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6385