Core hot wallet with (proposed) getxpub RPC

I'm working a Bitcoin Core PR that introduces a `getxpub` RPC: https://github.com/bitcoin/bitcoin/pull/22341

Along with some other changes, that would probably be better than the current hot wallet implementation.

On the Specter side, the flow should be to obtain xpub(s) from a core wallet, and then import the multisig descriptors into that same wallet. So it's a slight change from all the other devices where you create fresh watch-only core wallet.

It should probably not use the `specter/` path prefix. Although if the wallet is created by Specter than I guess that's fine.

Review the existing hot-wallet and device-wallet flow first, then compare it with the proposed Bitcoin Core getxpub RPC and the multisig-descriptor import flow described here. Done means the Specter-side flow can obtain xpubs from a Core wallet and import descriptors into that same wallet, with the specter/ path-prefix decision resolved.

Written by the indexing model from the issue text.