allow providing additional custom imports

[FyiurAmron]

Is your feature request related to a problem? Please describe.
Making a complete GraphQL app across separate schema files and modules is now tricky. Each type that is used externally needs to be manually remapped, which is extremely tedious, produces tons of boilerplate and is fault-prone for large integrations.

Describe the solution you'd like
Allowing adding imports to codegen files via e.g. extending the already existing DataModelMapper.getImports() to not start with empty HashMap but to populate it from a config var would make this trivial. It could be named e.g. just imports or additionalImports,

Describe alternatives you've considered
Injecting that text via other setting, e.g. generatedAnnotation - impossible due to leading @ and extremely hackish anyway.