add refresh_expires_in property to credentials

[Cantasura]

• Added the property refresh_expires_in property. This can help e.g. when the access token and refresh token is expired. With this property it is easier to handle the expiration of the refresh token.
  When the access token is expired and the lib tries to get a new access token with an expired refresh token, this will probably lead to and error.

---

• [✓] I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Contributions to our repos should follow the Dart style guide and use dart format.
• Most changes should add an entry to the changelog and may need to rev the pubspec package version.
• Changes to packages require corresponding tests.

Note that many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[Cantasura]

can somebody please review this? :)

[kevmoo]

@devoncarew ?

[mosuem]

As this repo has moved to https://github.com/dart-lang/tools/tree/main/pkgs/oauth2, please refile the PR there. Thanks!