Add optional model configuration

### 🔖 Enhancement description

Add a large model compatible with OpenAI protocol to the assistant

### 🎤 Pitch

.env:

_APP_ASSISTANT_OPENAI_API_KEY=YOUR_OPENAI_API_KEY
_APP_ASSISTANT_BASE_URL=https://api.openai.com/v1
_APP_ASSISTANT_MODEL_NAME=gpt-4o
_APP_ASSISTANT_EMBEDDING_MODEL=text-embedding-ada-002

### 👀 Have you spent some time to check if this issue has been raised before?

- [x] I checked and didn't find similar issue

### 🏢 Have you read the Code of Conduct?

- [x] I have read the [Code of Conduct](https://github.com/appwrite/.github/blob/main/CODE_OF_CONDUCT.md)