If we decide to keep this in primitives.hpp then also
select_outputs_algorithm should go there with it for consistency.

However primitives.hpp deals with basic data types that exist on the
protocol level. select_outputs_result is a very specific data structure
which is the utxos + change amount from select_outputs().

I want to move `select_outputs_result` struct back into transaction.hpp
underneath the `select_outputs_algorithm` enum.