feat: Get id token for impersonated service account

[gjvanahee]

Hello, thanks for a maintaining a great sdk. In order to autenticate impersonated service accounts with e.g. a Cloud Run service, the ISAC IdToken has to be fetched for a given target audience.

I have made this PR to allow this using the ImpersonatedServiceAccountCredentials class, so the AuthTokenMiddleware can use it to add the ISAC IdToken to a request. The change also allows ApplicationDefaultCredentials::getIdTokenMiddleware() and getIdTokenCredentials() to find ImpersonatedServiceAccountCredentials.

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[bshaffer]

Hi @gjvanahee ! This looks great, thank you for contributing this.

Coincidentally, I was just working on this in #580! We will get this merged soon, and then merge your additions as well.

[gjvanahee]

Hey @bshaffer, That is great to hear! Let me know if I can help out.

[bshaffer]

@gjvanahee If you'd like to help, you could use test #580 in your own use-case. I would love to have you review it!

composer require google/auth:dev-add-impersonated-service-account-credentials

I'll use the tests you've written in this PR as well to make sure it works the same.

Thanks again

[bshaffer]

Closing this PR as it should be completely implemented by #580