Service Account Impersonation

[lihan]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Added service account impersonation feature.

Required service account setup:
https://developers.google.com/identity/protocols/oauth2/service-account#delegatingauthority

[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.

[tswast]

@chalmerlowe @Linchin For your context when reviewing this feature request, here is the discussion the last time we considered adding this feature: #28

[Linchin]

Hi @lihan, thank you for your contribution, we appreciate it. There are a few things:

1. Could you please allow repo maintainers to change the PR, as described in this doc? Thus we can merge updates in main into your branch, or make necessary changes.
2. At the bottom of the page you can see the presubmit tests results. One of them called "conventionalcommits.org" did not pass, as we require the first commit of the PR to follow the same conventional commits requirements. You can change the commit message by an interactive rebase, and then force push to this branch. I know this may be a lot of info, so please let me know if you need more details.
3. I see that you have updated unit tests for parse_url() and it's great! Similarly, could you also add unit tests for the helper function and client code? We usually require 100% unit test coverage.

Feel free to let us know if you need any more info or help! We're glad to work with you.

[Linchin]

nit: assert the exact value, instead of Boolean value