[Vertex AI] Bypass proxy for testing against staging

[andrewheard]

Do not submit, for temporary testing against the Vertex AI staging API. To use, run gcloud auth print-access-token and set the printed value to the environment variable GCLOUD_ACCESS_TOKEN in Xcode.

cURL Example:

curl \
  -H "Authorization: Bearer $(gcloud auth print-access-token)" \
  -H "Content-Type: application/json" \
  'https://us-central1-staging-aiplatform.sandbox.googleapis.com/v1beta1/projects/{PROJECT_ID}/locations/us-central1/publishers/google/models/{MODEL_NAME}:generateContent' \
--data-binary @- << EOF
{
  "contents": [
    {
      "role": "user",
      "parts": { "text": "Why is the sky blue?" }
    }
  ]
}
EOF

#no-changelog