[POC/Draft] Add impersonated service account

[konalegi]

Hey folks, thank you for your awesome gem.

I've tried to implement service account impersonation with ADC. Although TBH I have no idea if I'm heading right direction. Could you please give me suggestions, if I'm even doing the right things?

Thank you!

[google-cla[bot]]

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.

[conventional-commit-lint-gcf[bot]]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot https://conventionalcommits.org/

[viacheslav-rostovtsev]

Hi! We got to implement the impersonated service account functionality recently. It ended up more broad than your user credential-linked version. Please TAL.