ZSA assets swap

[ConstanceBeguier]

This PR introduces the ability to create ActionGroup and SwapBundle for performing asset swaps.

This PR modifies the Builder:

• A new split_notes element has been added, which contains reference split notes. These notes can be used when creating actions from spend and output notes.
• A new build_action_group function allows the creation of an ActionGroup from a Builder. Like when creating a Bundle, the ActionGroup will initially be without proof and signatures. You will need to call create_proof and apply_signatures to add them.

This PR creates a new SwapBundle structure. A SwapBundle is created from a list of ActionGroups. During this creation, the following operations are performed

• The evaluation of the value balance of the SwapBundle (by summing the value balances of each ActionGroup).
• The evaluation of the binding signature key bsk of the SwapBundle (by summing the bsk of each ActionGroup).
• The binding signature of the SwapBundle which is equal to the signature of the SwapBundle SIGHASH with the binding signature key bsk.