OP_CTV support for elements

Copied from https://github.com/ElementsProject/elements/pull/949#issuecomment-870125117

Challenges:

- Directly migrating OP_CTV is tricky because fees in elements are output UTXO. The value cannot be known statically while receiving money in CTV controlled output. We would have to modify the template w.r.t to not include fee outputs.
- It might require more thinking about how the templating would interact with confidential assets. Just directly using the same template from bitcoin BIP 119 might not be the best way.
- All the functionality which OP_CTV allows is permitted by the current design. You can directly fix the transaction structure by looking at the outputs themselves. The solution is more generic and flexible than a fixed template. You can emulate any specific template that you want here.
- The best selling point about BIP119 is IMO the tooling around it(Sapio). But I don't think we have the bandwidth to extend/maintain Sapio for elements.

No se ha indicado ningún archivo fuente, prueba ni punto de entrada. Empieza revisando la discusión enlazada de BIP 119 y los desafíos enumerados relacionados con las comisiones y los activos confidenciales; aún es necesario definir los criterios de finalización para la compatibilidad con OP_CTV en Elements.

Escrito por el modelo de indexación a partir del texto del issue.