This would allow wallet operations like funding transactions with smart contract-based inputs such as LN, or any input the Core wallet doesn't know how to sign for.

https://github.com/bitcoin/bitcoin/pull/17211#issuecomment-933674310
#ifndef BITCOIN_WALLET_WALLET_H
#define BITCOIN_WALLET_WALLET_H
#include <consensus/amount.h>
#include <fs.h>
#include <interfaces/chain.h>
#include <interfaces/handler.h>
#include <outputtype.h>
#include <policy/feerate.h>
#include <psbt.h>
#include <tinyformat.h>
#include <util/message.h>
#include <util/strencodings.h>
#include <util/string.h>
#include <util/system.h>
#include <util/ui_change_type.h>
#include <validationinterface.h>
#include <wallet/coinselection.h>
#include <wallet/crypter.h>