Struct tag field to link to modules

[exavolt]

Currently, it goes something like

type Config struct {
  OneModuleToUse string
  Modules map[string]Module
}

Currently, we need to provide the modules through descriptor. It should be possible to declare the modules in OneModuleToUse. The loader and docgen will then resolve it automatically.

type Config struct {
  OneModuleToUse string `env:"ONE_MODULE_TO_USE,oneof=Modules"` // for this field, we'll refer to the field Modules
  Modules map[string]Module
}