Support provisioning instances without public IPs on GCP

[r4victor]

Similar to AWS, GCP backend config should support public_ips: true/false property:

type: gcp
  creds:
    type: default
  public_ips: false

To provision an instance without public IP in GCP, the instance should have no access config.

The user is expected to set up Cloud NAT that enables instances without public IPs to access the internet. dstack should probably check if Cloud NAT is set up.