google: support service account impersonation

[gIthuriel]

Adds support for service account impersonation when a URL for service account impersonation is provided.

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We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
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Patch Set 1:

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Message from Patrick Jones:

Patch Set 1:

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Message from Cody Oss:

Patch Set 1:

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Message from Cody Oss:

Patch Set 1:

(5 comments)

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This PR (HEAD: 975d095) has been imported to Gerrit for code review.

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Message from Patrick Jones:

Patch Set 1:

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Message from Cody Oss:

Patch Set 2:

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Patch Set 3:

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Patch Set 3:

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Message from Cody Oss:

Patch Set 4: Run-TryBot+1 Trust+1

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Message from Go Bot:

Patch Set 4:

TryBots beginning. Status page: https://farmer.golang.org/try?commit=9ec537ab

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Message from Go Bot:

Patch Set 4: TryBot-Result+1

TryBots are happy.

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Patch Set 4: Code-Review+2

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Patch Set 4:

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Message from Tyler Bui-Palsulich:

Patch Set 4: Trust+1

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Message from Patrick Jones:

Patch Set 4:

(3 comments)

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This PR (HEAD: 9c21878) has been imported to Gerrit for code review.

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Message from Cody Oss:

Patch Set 5: Run-TryBot+1

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Message from Go Bot:

Patch Set 5:

TryBots beginning. Status page: https://farmer.golang.org/try?commit=f74621a6

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This PR is being closed because golang.org/cl/285012 has been merged.