Shape Requirements for REST API

[julian-risch]

@Timoeller will drive the efforts around this roadmap topic starting from September.

Instead of jumping right into implementation, let's take the time to collect requirements and design a solution that addresses them.
Let's take into account the CLI could be the main entry point for the REST API.

Context

• Existing GitHub repo: https://github.com/deepset-ai/haystack-rest-api
• Blog post about serving Haystack with ray serve: https://haystack.deepset.ai/blog/run-haystack-pipelines-with-ray-serve
• Older roadmap topic: Make query endpoint ready for production workloads #3755
• Open issue on streaming handler feat: add generator of StreamingHandler into pipeline output. #4890
• User request for REST API examples Are there any better examples for running the rest-api WITHOUT docker? The explanation given on the rest-api page is really bad. #5693