[Feature Request] Add support for OpenAI text-embedding-3-small and text-embedding-3-large models in Vector Search with Firestore

### [READ] Step 1: Are you in the right place?

Issues filed here should be about a feature request for a **specific extension in this repository**. To file a feature request that affects multiple extensions or the Firebase Extensions platform, please reach out to
[Firebase support](https://firebase.google.com/support/troubleshooter/report/features/) directly.

### [REQUIRED] Step 2: Extension name

This feature request is for extension: **Vector Search with Firestore**

### What feature would you like to see?

Currently, when OpenAI is selected as the embedding provider, the extension uses the older model `text-embedding-ada-002`.
OpenAI has since released improved models (`text-embedding-3-small` and `text-embedding-3-large`) that provide better performance, efficiency, and cost-effectiveness.

I would like to request the ability to configure which embedding model is used within the extension settings. Ideally, the options should include the newer `text-embedding-3-small` and `text-embedding-3-large` models, while keeping backwards compatibility with `text-embedding-ada-002`.

### How would you use it?

Allowing the selection of newer embedding models would:
- Improve embedding quality and retrieval accuracy.
- Provide cost optimization (developers can choose between the small and large models based on their use case).
- Future-proof apps using this extension by aligning with OpenAI’s recommended embedding models.

In my app, this would directly enhance the performance of semantic search and retrieval-augmented generation (RAG) workflows, leading to better end-user experience.