Prioritize custom type conversion over standard conversions

As discussed with @erikwrede we have to make sure that we prioritize registered custom type conversions over the defaults.

Example:

This:

```python
from sqlalchemy_utils import JSONType

@convert_sqlalchemy_type.register(JSONType)
def convert_column_to_json(*args, **kwargs):
return graphene.types.generic.GenericScalar
```

will result in a JSONString field instead of a GenericScalar