gce: Add support for publishing Service Account Issuer documents to GCS

[sl1pm4t]

This change adds for support for GCS as a target for cluster.spec.serviceAccountIssuerDiscovery.discoveryStore.

The generated service account issuer URL will then be in the form:
https://storage.googleapis.com/<BUCKET>
meaning the JWKS URI becomes:
https://storage.googleapis.com/<BUCKET>/openid/v1/jwks

This is the first step towards supporting Workload Identity on GCP kOps clusters.

[k8s-ci-robot]

Hi @sl1pm4t. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[hakman]

/ok-to-test
/cc @justinsb
/assign @justinsb

[justinsb]

Thank you @sl1pm4t - looks great!

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: justinsb

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