adding data source for identity pool

[modular-magician]

follow up to PR #4071 and part of bigger issue hashicorp/terraform-provider-google#7455

If this PR is for Terraform, I acknowledge that I have:

• 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)

`google_iam_workload_identity_pool`

Derived from GoogleCloudPlatform/magic-modules#4134