feat: fetch discrete orders for conditional order of generator type

[mfw78]

Problem

Some conditional orders, such as TWAP, are known as "generator" types in ComposableCoW lingo. That is, they are a conditional order that produces a known set of discrete orders. User interfaces that want to track discrete order settlement effectively have to reproduce the TWAP logic to generate these discrete orders for monitoring.

Solution

Define an abstract discreteOrders function that all generator conditional order types implement.