GW2024 Part 1: GW2024 extends MigrationType

[shtukas]

This is the first of a series of changes in which we are going to implement the Guardian Weekly 2024 migration.

Unlike previous migrations implemented in essentially a single pull request, see for instance the example of Newspaper2024 ( #964 ), because GW2024 is also a teaching opportunity with various documentation having already been written and new ones coming soon, the question of how to present the migration code update in the engine was posed.

This PR is the first PR in the series. A migration ultimately starts with the MigrationType trait being extended. The change is the least amount of code that needed to be written to make it compile and pass the tests (note that no tests actually needed to be changed). Moreover we make a small change in the handle function of the Estimation handler to prevent it to run. This is in case somebody wants to run the state machine.

Note that a PR to introduce the GW2024Migration object/name space had actually already been made #999.

It's interesting to notice that two functions have been introduced, but not implemented: GW2024Migration.priceData and GW2024Migration.zuoraUpdate. They are the main function used in the Estimation step and the Amendment step. The engine no longer makes the assumption that there is a default implementation for them. We are purposely leaving them un implemented because they are not going to be called (even if somebody runs the GW2024 state machine).

The next PRs of this series will show the modifications required to activate the lambdas one by one.