Merge bitcoin#31243: descriptor: Move filling of keys from DescriptorImpl::MakeScripts to PubkeyProvider::GetPubKey

acee5c5 descriptors: Have GetPrivKey fill keys directly (Ava Chow)
4b03031 descriptors: Move FlatSigningProvider pubkey filling to GetPubKey (Ava Chow)
25a3b9b descriptors: Have GetPubKey fill origins directly (Ava Chow)
6268bde descriptor: Remove unused parent_info from BIP32PUbKeyProvider::GetPubKey (Ava Chow)
0ff072c wallet, rpc: Only allow keypool import from single key descriptors (Ava Chow)

Pull request description:

  Instead of having `MakeScripts` infer what pubkeys need to go into the output `FlatSigningProvider`, have each of the `PubkeyProviders` that have `GetPubKey` and `GetPrivKey` called fill it directly with relevant keys and origins.

  This allows for keys and origins to be added that won't directly appear in the output, which is necessary for `musig()` descriptors.

  Split from bitcoin#29675

ACKs for top commit:
  fjahr:
    Code review ACK acee5c5
  theStack:
    re-ACK acee5c5
  rkrux:
    ACK acee5c5

Tree-SHA512: c1841359bcb08cdd433122deef96579236928660785f3357a3eb584e47d290cd1c60ebe8f7fba50f178ba45c9a90773124e0f509e36c5a0df97c1a4890e03e5c

Author: glozow