fix: Create separate firewall rule for egress to TPUs

[tomasgareau]

Addresses #1124.

I bumped into this when trying to create a safer-cluster in a VPC network with a default deny egress rule. The intra_egress firewall rule was required to allow the default pool to report back to the control plane so it could be deleted (as part of the google_container_cluster.primary create operation).

However, the intra_egress firewall rule had a Terraform dependency on the google_container_cluster.primary resource, so it was never created, preventing the cluster from creating successfully.

This PR simply creates a separate rule for egress to the tpu_ipv4_cidr_block, allowing the intra_egress rule to get created before the google_container_cluster resource.

[comment-bot-dev]

Thanks for the PR! 🚀
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