feat: Add support for custom package repositories

Description

Hi there 👋

According to https://dart.dev/tools/pub/custom-package-repositories, you're able to use pub dependencies hosted on repositories that are not pub.dev using the dart pub token add _____ command. Unfortunately, our workflows don't seem to support this credential type.

It would be awesome if we could support it, but I see a few big questions:

1. You'd only want to run that command if you have tokens to pass in. (should be accomplishable with a if statement on that step.)
2. You'd want to be able to pass in a list of tokens in the off chance you're using multiple pub dependencies. (Not sure if this is possible.)
3. Each token needs the token itself AND the site it applies to, so a simple string probably won't work. Maybe a map with two values?

Not sure how we can support this, but since it's a part of the Dart CLI it's probably important to at least consider it. I personally ran into this issue working on a side project and it's led me away from the workflows.

Happy to chat further :)

Requirements

• All CI/CD checks are passing.
• There is no drop in the test coverage percentage.

Additional Context

No response

Start by reviewing the repository's reusable workflow definitions and how they currently provide credentials for Dart or pub dependencies. Check the Dart CLI behavior for dart pub token add, including conditional execution and multiple repository/token pairs. Done means custom package repository tokens are supported without exposing secrets, and CI/CD checks and coverage remain passing.