Following #17261, the way to sign transactions, PSBTs, and messages was to use GetSigningProvider() and get a SigningProvider containing the private keys. However this may not be feasible for future ScriptPubKeyMans, such as for hardware wallets. Instead of exporting a SigningProvider containing private keys, we need to pass these things into the ScriptPubKeyMan (via CWallet) so that they can do whatever is needed internally to sign them. This is largely a refactor as the logic of processing transactions, PSBTs, and messages for is moved into LegacyScriptPubKeyMan and CWallet instead of being handled by the caller (e.g. signrawtransaction).
To help with this, I’ve refactored the 3(!) implementations of a SignTransaction() function into one generic one. This function will be called by signrawtransactionwithkey and LegacyScriptPubKeyMan::SignTransaction(). CWallet::CreateTransaction() is changed to call CWallet::SignTransaction() which in turn, calls LegacyScriptPubKeyMan::SignTransaction(). Other ScriptPubKeyMans may implement SignTransaction() differently.
FillPSBT() is moved to be a member function of CWallet and the psbtwallet.cpp/h files removed. It is further split so that CWallet handles filling the UTXOs while the ScriptPubKeyMan handles adding keys, derivation paths, scripts, and signatures. In the end LegacyScriptPubKeyMan::FillPSBT still calls SignPSBTInput, but the SigningProvider is internal to LegacyScriptPubKeyMan. Other ScriptPubKeyMans may do something different.
A new SignMessage() function is added to both CWallet and ScriptPubKeyMan. Instead of having the caller (i.e. signmessage or the sign message dialog) get the private key, hash the message, and sign, ScriptPubKeyMan will now handle that (CWallet passes through to the ScriptPubKeyMans as it does for many functions). This signing code is thus consolidated into LegacyScriptPubKeyMan::SignMessage(), though other ScriptPubKeyMans may implement it differently. Additionally, a SigningError enum is introduced for the different errors that we expect to see from SignMessage().
Lastly, GetSigningProvider() is renamed to GetPublicSigningProvider(). It will now only provide pubkeys, key origins, and scripts. LegacySigningProvider has it’s GetKey and HaveKey functions changed to only return false. Future implementations should return HidingSigningProviders where private keys are hidden.
Other things like dumpprivkey and dumpwallet are not changed because they directly need and access the LegacyScriptPubKeyMan so are not relevant to future changes.
