Add general support for proxies

[weilandia]

For example, supporting Helicone

curl --request POST \
  --url https://gateway.helicone.ai/v1/projects/your-project-id/locations/your-location/publishers/google/models/model-name:streamGenerateContent \
  --header 'Authorization: Bearer ${GCLOUD_API_KEY}' \
  --header 'Content-Type: application/json' \
  --header 'Helicone-Auth: Bearer ${HELICONE_API_KEY}' \
  --header 'Helicone-Target-URL: https://${LOCATION}-aiplatform.googleapis.com' \
  --header 'User-Agent: curl/7.64.1' \
  --data '{
  "contents": {
    "role": "user",
    "parts": {
      "text": "Which theaters in Mountain View show Barbie movie?"
    }
  },
  "generation_config": {
    "maxOutputTokens": 1
  }
}'

[timabdulla]

#27 addresses this, if you want to try my branch.