Support dataset upsert logic

What does this merge request do and why?

Small MR to avoid raising exceptions when adding new examples to the existing dataset.

How to set up and validate locally

1. Check out to this merge request's branch.
2. Install dependencies.

   poetry run install

3. Run:

   poetry run eli5 datasets create duo_workflow.fix-broken-pipeline.1 datasets/duo_workflow/fix-broken-pipeline-v1 --upsert # success
   poetry run eli5 datasets create duo_workflow.fix-broken-pipeline.1 datasets/duo_workflow/fix-broken-pipeline-v1 # fail, the dataset exists already 

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.

Blocked by !85 (merged)