Have a standard way to run custom codegen plugins built with Unreal #24
Description
Running custom codgen plugins built with the unreal build tool can be kind of awkward. You want to run them in a prebuild step for your project but they are built after the prebuild steps. This works fine for the Ecsact Unreal plugin because it gets installed as an engine plugin and is pre-built, but for custom in-project ecsact codegen plugins it's not very nice. At best you have to compile twice and if you mess up the code generation you can be in a frustrating situation where the generated code is breaking your build but you can't re-build.
The solution might be to only allow engine plugins and require users to have their own custom build steps. Then we need a "standard" way to discover these custom ecsact codegen plugins.