Look into using promotions from the API

Problem/Motivation

When this module was first created, the Vertex AI API didn't have any support for promotions. The Vertex AI Search Promoted Results → module was written to fill the void, but now it looks like it's part of the API.

https://cloud.google.com/generative-ai-app-builder/docs/configure-servin...

Proposed resolution

We should look at implementing the promotion functionality as part of this module, rather than a separate one.

We can keep the other module, but if this provides all the same functionality straight from the API, that could be simpler.

Remaining tasks

1. Look into how the promotions work in the API.
2. Determine how promotion data is stored.
3. Make POC for displaying promotion data into Drupal.

User interface changes

The search results page will have a "promoted" section. Templates will need updated.

API changes

The changes are to Google's API, but we'll need to update our implementation to work with them.