[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
• Support Async UDFs as input to aggregations #17619
• Join expression
• Async UDAF
• Async UDWF
• Async UDTF
• Refactor Async UDF rewriting in physical planner #18150
• Support ser/de of AsyncFuncExec #19112
• Implement metrics for AsyncFuncExec #19623
• Nesting async UDF calls causes an internal error #20031

[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.

[Jefffrey]

I've raised #18150 as I was looking at the physical planner code and was quite confused by the async rewriting when investigating a bug (#18149) and I see there was some discussion about this in the original PR, so hopefully we can continue that discussion.

I've also added some tasks to the epic here that were mentioned in the original PR (join expressions, async udafs, etc.)

I won't create issues for them yet as I think we want to handle the rewrite issue first, otherwise we'd be adding more of that rewrite logic into the physical planner 🤔