`rawissueasset` (and other issuance RPCs) should not allow issuing asset from a legacy output

In https://github.com/ElementsProject/elements/blob/18fadbafcceff70a7d328ca739e09004dc51e37a/src/confidential_validation.cpp#L282-L284 we check, when issuing assets, that a transaction has witness data attached to it, which can result in confusing errors when users try to issue assets starting from legacy outputs.

In general we want to discourage legacy outputs anyway except when absolutely necessary (e.g. when using signmessage, since we do not yet have a BIP322 implementation for Elements), since they are more costly to the user, addresses have worse checksumming properties, and they are more complex for implementors to work with. Narrowing this RPC will nudge us in that direction while also replacing a confusing footgun with a clear error message.

Empieza en src/confidential_validation.cpp, en las líneas 282-284, y luego sigue rawissueasset y los demás puntos de entrada de las RPC de emisión para ver cómo las salidas heredadas llegan a la validación. Se considera terminado cuando las RPC de emisión rechacen las salidas heredadas con un error claro en lugar del confuso error de datos de witness, mientras que la ruta de emisión permitida siga sin verse afectada.