Fix id token datasource

[upodroid]

Fixes: hashicorp/terraform-provider-google#7333

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• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
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Release Note Template for Downstream PRs (will be copied)

iam: fixed `google_service_account_id_token` datasource to work with User ADCs and Impersonated Credentials

[modular-magician]

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@c2thorn, please review this PR or find an appropriate assignee.

[upodroid]

I ran the TestAccDataSourceGoogleServiceAccountIdToken_impersonation test to verify the behaviour of the change and it works with User ADCs.

Also, can you add the hacktoberfest-accepted label to the PR? Thanks

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 4 files changed, 10 insertions(+), 22 deletions(-))
Terraform Beta: Diff ( 4 files changed, 10 insertions(+), 22 deletions(-))

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 4 files changed, 10 insertions(+), 22 deletions(-))
Terraform Beta: Diff ( 4 files changed, 10 insertions(+), 22 deletions(-))

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 4 files changed, 11 insertions(+), 18 deletions(-))
Terraform Beta: Diff ( 4 files changed, 11 insertions(+), 18 deletions(-))