Support instances without public IP for GCP

[smokfyz]

Closes #1321

[smokfyz]

How to test?

1. Go to https://console.cloud.google.com/hybrid/routers/list?referrer=search&authuser=2&project=dstack
2. Create router

3. Go to https://console.cloud.google.com/net-services/nat/list?authuser=2&project=dstack
4. Create NAT

5. Deploy local dstack server with config:

projects:
  - name: main
    backends:
      - type: gcp
        project_id: dstack
        creds:
          type: default
        public_ips: true
        regions: [europe-west1]

6. Create dev environment with current branch dstack run . -f ./dev.dstack.yml

# dev.dstack.yml content
type: dev-environment
python: "3.11"
ide: vscode

7. Using dev environment configure server on the remote server

curl -O https://dl.google.com/dl/cloudsdk/channels/rapid/downloads/google-cloud-cli-480.0.0-linux-x86_64.tar.gz
tar -xf google-cloud-cli-480.0.0-linux-x86_64.tar.gz
./google-cloud-sdk/install.sh
./google-cloud-sdk/bin/gcloud init
./google-cloud-sdk/bin/gcloud auth application-default login
export GOOGLE_CLOUD_PROJECT=dstack
pip install -e ".[all]"
apt install vim -y
mkdir -p ~/.dstack/server && vim ~/.dstack/server/config.yml
dstack server --no-default

# ~/.dstack/server/config.yml
projects:
  - name: main
    backends:
      - type: gcp
        project_id: dstack
        creds:
          type: default
        public_ips: false
        regions: [europe-west1]

11. Deploy test workload dstack run . -f ./test.dstack.yml

mkdir test && cd ./test
dstack config --url http://127.0.0.1:3000 --project main --token <server_token>
dstack init
vim test.dstack.yml
dstack run . -f ./test.dstack.yml

Workload example:

# ./test.dstack.yml
type: task

python: 3.11
backends: [gcp]
commands:
  - echo "Hello, world!" && sleep 10 && echo "Bye!"

[r4victor]

Looks good!