Expose search output as an API endpoint

I wonder if we could leverage what https://www.drupal.org/project/jsonapi_resources → is using to create a custom JSON:API endpoint, rather than our own custom endpoint. Not sure if search results translates well into JSON:API's structure, but it could be worth investigating.

If we wanted to keep it really simple, we could just pass along the JSON that Vertex gives us, without any transformation.

Closing in favor of : https://www.drupal.org/project/vertex_ai_search/issues/3507216 ✨ Create a REST Resource for Vertex Search Active