PoC: Vertex AI Search endpoint for Documentation tool of Duo Chat (AI Gateway)

What does this merge request do and why?

Related to Draft: Retrieval Augmented Generation (RAG) for... (gitlab-org/gitlab!142787 - closed)

How to set up and validate locally

curl -X 'POST' \
  'http://0.0.0.0:5052/v1/search/docs' \
  -H 'accept: application/json' \
  -H 'Content-Type: application/json' \
  -d '{
    "type": "string",
    "metadata": {
      "source": "string",
      "version": "17.0.0-pre"
    },
    "payload": {
      "query": "how to create an issue?",
      "params": {
        "page_size": 10,
        "filter": ""
      }
    }
}'

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.