Table functions: add push down projections support

[alexeyzimarev]

DuckDB C API allows push-down projections, it's be great to get it supported here as well.

[Giorgi]

I didn't add support for it initially because I wanted to wait for the feedback. I can add support for projection push-downs but it will require a third callback making the RegisterTableFunction a bit more complex.

[alexeyzimarev]

I can confirm that table functions work very well :) Apart from the projection push-down not being available, the other missing thing is limited type support for returned values. Here I found that composite types are fully supported, but I don't think it works with table functions. I am not sure as I only briefly checked the code, will spend a bit more time in the coming days.

[Giorgi]

Reading composite types is supported for every type except Union. Writing is supported for List and Array only.

[alexeyzimarev]

Yeah, I only meant what a table function can return.

[Giorgi]

Table function can return anything as long as WriteValue is implemented for the properties that you want to write. At the moment the writer supports primitive types, Enum, List and Array.

One workaround for complex properties is to write the nested properties to several columns of the result set instead of one column.