feat(gcp): add Workload Identity Pool icon

[kakakakakku]

Hi 😀

The motivation for this pull request is to add the Google Cloud Workload Identity Pool icon.

Check ✔️

I checked my sample code.

if __name__ == "__main__":
    from diagrams import Diagram
    from diagrams.gcp.security import WorkloadIdentityPool

    with Diagram("", filename="/tmp/test_wip", show=False):
        node = WorkloadIdentityPool("WorkloadIdentityPool")

Could you please review? Thanks a lot 👍