[GCP] Add gcp.force_enable_external_ips configuration

[Ultramann]

This PR adds a new configuration to the ~/.sky/config.yaml for gcp, force_enable_external_ips. Setting this configuration to true (default is false for backwards compatibility) will cause the gcp subnet provisioner to create VMs with a public ip address even if use_internal_ips is set to true; this is useful when communication within a vcp is desired and the VM needs to make calls to the public internet.

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
• All smoke tests: pytest tests/test_smoke.py
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• Backward compatibility tests: conda deactivate; bash -i tests/backward_compatibility_tests.sh

[Michaelvll]

Thanks for adding the support of this new knob @Ultramann! LGTM.

nit: to make it expose to users, let's add it to our docs as well: https://github.com/skypilot-org/skypilot/blob/master/docs/source/reference/config.rst

[Michaelvll]

Thanks for the update @Ultramann! This looks good to me. Once we have tests added, it should be good to go.

[Michaelvll]

Suggested change

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This cause the failure of CI ; )