google: add DelegateTokenSource

[salrashid123]

Delegate Credentials is basically a way for a user or (more common) service account to impersonate another service account.

Ref:

https://cloud.google.com/iam/docs/service-accounts#the_service_account_token_creator_role
https://cloud.google.com/iam/docs/creating-short-lived-service-account-credentials
https://cloud.google.com/iam/credentials/reference/rest/v1/projects.serviceAccounts/generateAccessToken

I've verified the sample with service_account, user_credentials and with GCE metadata server (and also delgate of delgate in squence)

Fixes #336

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[salrashid123]

/comments off

[salrashid123]

rename usage from DelegateTokenSource -> ImpersonatedTokenSource.

Seems to fit better w/ the distinction described here

[salrashid123]

Also renamed the parameters to source and target to make it more aligned with
the python PR thats merged:

• Add delegate_credentials googleapis/google-auth-library-python#299

i didn't rename the tokens source since source is already in the name...

[salrashid123]

@bradfitz Could someone kickoff a review on this? I think should be relatively straightfoward and I've documented the system test steps via gcloud here:

• Add delegate_credentials googleapis/google-auth-library-python#299 (comment)

Its not urgent but i do have the python-auth library support live know and i'm currently submitting one for java-auth separately.

[salrashid123]

@bradfitz Could we resume the review on this PR? (the java and python version are actually live now in the library set). I think the main outstanding issue is how to deal w/ the context thats passed in.