[EPIC] More Async User Defined Function work

[alamb]

In #6518 and #14837, @goldmedal introduced async user defined functions. This ticket captures follow on work to improve the functionality:

• [Async UDF] Add high level context / examples for user defined async functions  #16521
• Update AsyncScalarUDFImpl API to match ScalarUDFImpl API #16522
• [Blog] Async Scalar User Defined Functions #16525

[alamb]

@goldmedal is there any other todo items you can think of for async udfs?

[goldmedal]

I only implemented the function expression in Projection and Filter. Maybe we can implement for other plans, like the condition of Join. Then, we only currently support async scalar functions. I think async aggregation, window function, or table function can also be supported.