Support "stop" parameter in inference providers

[terrytangyuan]

🚀 Describe the new functionality needed

"stop" parameter: https://platform.openai.com/docs/api-reference/completions/create#completions-create-stop

💡 Why is this needed? What if we don't build it?

Only vLLM inference provider is supported/tested through https://github.com/meta-llama/llama-stack/pull/1715

Other thoughts

No response