Wire up Azure OpenAI completion provider (#8646)

This PR wires up support for [Azure
OpenAI](https://learn.microsoft.com/en-us/azure/ai-services/openai/overview)
as an alternative AI provider in the assistant panel.

This can be configured using the following in the settings file:

```json
{
  "assistant": {
    "provider": {
      "type": "azure_openai",
      "api_url": "https://{your-resource-name}.openai.azure.com",
      "deployment_id": "gpt-4",
      "api_version": "2023-05-15"
    }
  },
}
```

You will need to deploy a model within Azure and update the settings
accordingly.

Release Notes:

- N/A