Add provisioned throughput support in Google Vertex AI in JS SDK (custom headers)

Google has provisioned throughput.

Currently it’s impossible to use it in the JS SDK.

Please add ability to add custom headers to vertex AI requests.

curl -X POST
-H “Authorization: Bearer $(gcloud auth print-access-token)”
-H “Content-Type: application/json”
-H “X-Vertex-AI-LLM-Request-Type: dedicated” \ # Options: dedicated, shared
$URL
-d ‘{“contents”: [{“role”: “user”, “parts”: [{“text”: “Hello.”}]}]}’