Move key and script filling and signing from CWallet::FillPSBT to ScriptPubKeyMan::FillPSBT

Instead of fetching a SigningProvider from ScriptPubKeyMan in order
to fill and sign the keys and scripts for a PSBT, just pass that
PSBT to a new FillPSBT function that does all that for us.

src/wallet/test/psbt_wallet_tests.cpp

@@ -73,9 +73,7 @@ BOOST_AUTO_TEST_CASE(psbt_updater_test)
 
     // Try to sign the mutated input
     SignatureData sigdata;
-    psbtx.inputs[0].FillSignatureData(sigdata);
-    const std::unique_ptr<SigningProvider> provider = m_wallet.GetSigningProvider(ws1, sigdata);
-    BOOST_CHECK(!SignPSBTInput(*provider, psbtx, 0, SIGHASH_ALL));
+    BOOST_CHECK(spk_man->FillPSBT(psbtx, SIGHASH_ALL, true, true) != TransactionError::OK);
 }
 
 BOOST_AUTO_TEST_CASE(parse_hd_keypath)