Store TaprootBuilder in SigningProviders instead of TaprootSpendData

TaprootSpendData can be gotten from TaprootBuilder, however for PSBT, we
also need TaprootBuilders directly (for the outputs). So we store the
TaprootBuilder in the FlatSigningProvider and when the TaprootSpendData
is needed, we generate it on the fly using the stored builder.

src/script/descriptor.cpp

@@ -882,7 +882,7 @@ protected:
         if (!xpk.IsFullyValid()) return {};
         builder.Finalize(xpk);
         WitnessV1Taproot output = builder.GetOutput();
-        out.tr_spenddata[output].Merge(builder.GetSpendData());
+        out.tr_trees[output] = builder;
         out.pubkeys.emplace(keys[0].GetID(), keys[0]);
         return Vector(GetScriptForDestination(output));
     }