[backend:improvement] Support for async functions

[kkampli-singlestore]

This PR adds a check to see if the UDF is async or not in the vector input UDF call.

1. If the func is async, await to call it directly.
2. If the func is sync, run it in a separate thread and await the result.

[kesmit13]

FYI, there is another branch (cancel_udf) that implement async functions already. It also allows for non-async functions to be cancelled if the connection to the database engine is broken.

[kesmit13]

The PR is #64 . It just needs to be tested a bit more.

[srinathnarayanan]

The PR is #64 . It just needs to be tested a bit more.

This MR is pretty big for us to test end to end, @kkampli-singlestore can you maybe create a smaller MR with just kevin's Async function changes? thanks!

[kesmit13]

Async support has been addded in 9c649cf