Fix: importmulti only imports origin info for PKH outputs #15749
Since #15263, pubkeys are no longer imported for non-PKH (or WPKH, or any wrapped form of those) outputs, as that would incorrectly mark outputs to single-key versions of multisig policies as watched.
As a side effect, this change also caused origin info not to be imported anymore for multisig policies.
Fix this by plumbing through the full pubkey information for origins in FlatSigningProvider, and then importing all origin info we have in
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