Port `model_id` configuration parameter (`string`) to new `model` type in training workers

Closes #27 (closed)

Tested with these processes:

1. https://demo.arkindex.org/process/e2576c7e-bf9f-463a-8a8d-b73c27b0c5fe/0
2. https://demo.arkindex.org/process/033ac084-ba32-4690-893b-0f265d47b29a/0

Which produced these model versions on the correct models:

1. https://demo.arkindex.org/model-version/4a0e5a0f-5e76-4509-a6db-ee656d13b344
2. https://demo.arkindex.org/model-version/a5237a37-3f82-401c-a2e5-9662f0648486