Support custom OpenAI apis with a DSN

Using a DSN like symfony mailer/messenger. For example, for together:

```
openai://:${TOGETHER_API_KEY}@api.together.xyz/v1/chat/completions?model=meta-llama%2FLlama-3-70b-chat-hf&strategy=json
together://:${TOGETHER_API_KEY}@meta-llama%2FLlama-3-70b-chat-
```