Support private PyPI repositories

This issue is related to the following conversation from fast.ai forum: https://forums.fast.ai/t/nbdev-for-private-pypi-repositories-and-poetry-package-management/100523

# Goal of the issue

1. To gauge the interest in nbdev supporting private PyPi repos (e.g. GCP Artifact Registry).
2. To discuss the best way to integrate private PyPi repo support into nbdev.

If there's interest, I'd be happy to make the contribution.

# Problem

nbdev does not support dependencies from private PyPi repositories.

# Workaround

Changing the following line in [nbdev CI workflow](https://github.com/fastai/workflows/blob/master/nbdev-ci/action.yml):

```bash
test -f setup.py && pip install -e ".[dev]"
```

to:

```bash
test -f setup.py && pip install --extra-index-url https://$PYPIUSER:$PYPIPASSWORD@$PYPISERVER -e ".[dev]"
```

fixes the issue.

Inizia con il nbdev CI workflow collegato e il relativo comando `pip install -e ".[dev]"`, quindi esamina la conversazione sul forum fast.ai relativa all’ambito proposto. Un contributo richiederebbe un punto di integrazione concordato e criteri di accettazione per l’installazione delle dipendenze da repository PyPI privati; l’issue attualmente non definisce né l’uno né gli altri.

Scritto dal modello di indicizzazione a partire dal testo della issue.