Support for private repositories for pub

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Feature description

Support for private repositories for pub

When enabling dependabot support for a pub project there should be a way to configure the credentials for accessing private third-party repositories.

From discussion here: https://github.com/orgs/community/discussions/14200#discussioncomment-4050762

The authentication pub uses is documented here:
https://github.com/dart-lang/pub/blob/master/doc/repository-spec-v2.md#authentication

Basically dependabot needs to run `dart pub token add ` and give it a credential token on stdin for each private repository.

It will then pass that token to the repository in a 'Authorization: Bearer ' header when doing version listings and downloading packages.

Start by locating Dependabot's pub package-manager entry point and read the linked pub repository authentication specification. Trace how private repository credentials could be configured, then verify that each repository can receive its token through `dart pub token add ` and that the requested Authorization header behavior is covered. Done means private third-party pub repositories can be accessed during dependency updates.

Written by the indexing model from the issue text.