Inject PyPI information (URL, username, credentials) automatically into the workbench

### Checks

- [x] I have searched the [existing issues](https://github.com/kubeflow/notebooks/issues).
- [x] My request is related to one of the components in the [`kubeflow/notebooks`](https://github.com/kubeflow/notebooks) repository.

### Motivation

We're running internal PyPI repositories and we need to have the ability to inject these repo information (URL, username, credentials) automatically in the workbench environment so that pip installing from them works out of the box.

This was previously filled as

* https://github.com/kubeflow/kubeflow/issues/5210

It appears to largely duplicate

* https://github.com/kubeflow/notebooks/issues/239

Also somewhat related issues

* https://github.com/kubeflow/kubeflow/issues/7332
* https://github.com/kubeflow/kubeflow/issues/313

### Implementation

This opens up the auth inside a workspace can of worms.

### Are you willing & able to help?

- [x] I am able to submit a PR!
- [x] I can help test the feature!

No files, tests, or entry points are named in the issue. Review the linked duplicate issue 239 and related issues first, then clarify the authentication design and the supported PyPI URL, username, and credential injection behavior before implementation.

Written by the indexing model from the issue text.