Better async requests

What does this merge request do and why?

This MR uses aiohttp to manage async requests. Compared to the current way, this approach 2 major benefits

• Better performance.
  • Uses a session as a best practice. Current we are using 1 session per request which has some performance hit. For details see doc: https://docs.aiohttp.org/en/stable/http_request_lifecycle.html#using-a-session-as-a-best-practice.
  • Use coroutine instead of threading. Current we use asyncio.to_thread, in this MR we use asyncio.create_task.
• Significantly simplify the ModelHandle trait implementation. All the common async logic are abstracted away into a separate function which uses aiohttp. This means that each concrete implantation becomes much simpler, we just need to specify the header, json payload and response parsing. Adding a new model becomes simpler now.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Merge request checklist

• I've ran the affected pipeline(s) to validate that nothing is broken.
• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.