Generator - support 3rd party annotations

[vaind]

Currently, the generator omits (doesn't store) an entity field with an unknown annotation.
From generator.dart:

            // skip unknown annotations
            print(
                "warning: skipping field '${f.name}' in entity '${element.name}', as it has the unknown annotation type '$annotType'");
            continue;
          }

Instead, we should process those fields normally and ignore unknown annotations.

[vaind]

The original code was incorrect, it wasn't checking annotation type but property type, thus this is a non-issue.