blinding RPC enhancements

We want to support a couple additional abilities:

1) Blind with excess, and be able to consume it. This matters in coinjoin scenarios when people are blinding their transaction to make sure that the inputs/outputs can't be matched up. This is quite similar to the MimbleWimble "excess" value.

2) We should have a method of passing in asset blinders and types to allow a surjection proof to be made for an output we own, but not the corresponding input. This matters in the DG demo scenario when people doing an exchange don't have access to the thing they are trying to exchange to.

El issue no menciona archivos, tests ni puntos de entrada. Comienza localizando la implementación existente del blinding RPC y sus tests, y después aclara la API y la semántica de las pruebas para consumir blinders excedentes y proporcionar blinders y tipos de assets. Se considera terminado cuando ambas capacidades solicitadas están especificadas, implementadas y cubiertas por tests.

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