Support completions API

[nstogner]

Supporting openai-api-compatible endpoints like the /v1/chat/completions and /v1/completions APIs would have the following benefits:

• Allow Jetstream to be used as a drop-in replacement for the vLLM server
• Make it easier to demo Jetstream with OSS chat UIs (i.e. https://github.com/open-webui/open-webui, https://github.com/danny-avila/LibreChat, etc)