Design REST API

[julian-risch]

Instead of jumping right into implementation, let's take the time to collect requirements and design a solution that addresses them. Related issue about streaming: https://github.com/deepset-ai/haystack/issues/4890 Related issue about deployment guides: https://github.com/deepset-ai/haystack/discussions/5693 Related roadmap item from Q4 2023: https://github.com/deepset-ai/haystack/issues/5266