Fix duckdb/duckdb#13993 - avoid disabling optimizers for SET VARIABLE (duckdb/duckdb#14028)
Proper NULL handling in special json extraction functions (duckdb/duckdb#14032)
Adds Julia support for scalar UDFs (duckdb/duckdb#14024)

Fix duckdb/duckdb#13993 - avoid disabling optimizers for SET VARIABLE (duckdb/duckdb#14028)
Proper NULL handling in special json extraction functions (duckdb/duckdb#14032)
Adds Julia support for scalar UDFs (duckdb/duckdb#14024)

Co-authored-by: krlmlr <krlmlr@users.noreply.github.com>

This PR improves the Julia support for user defined UDFs.

It adds the `ScalarFunction` type and the `@create_scalar_function`
macro that automatically generates a DuckDB compatible wrapper. See also
discussion #13176 for details.

Numeric types, dates, strings, missing values and exceptions are
supported. Composite types are not yet implemented.

## Example Usage:

```julia
using DuckDB, DataFrames

f_add = (a, b) -> a + b
db = DuckDB.DB()
con = DuckDB.connect(db)

# Create the scalar function 
# the second argument can be omitted if the function name is identical to a global symbol
fun = DuckDB.@create_scalar_function f_add(a::Int, b::Int)::Int f_add
DuckDB.register_scalar_function(con, fun)

df = DataFrame(a = [1, 2, 3], b = [1, 2, 3])
DuckDB.register_table(con, df, "test1")

result = DuckDB.execute(con, "SELECT f_add(a, b) as result FROM test1") |> DataFrame
```

## Performance

The performance of the auto-generated wrapper is comparable to pure
DuckDB/Julia. I measured the elapsed time (in seconds) of adding 10
million numbers in a coarse benchmark:

|               | Int         | Float       |
| ------------- | ----------- | ----------- |
| DataFrames.jl | 0.092947083 | 0.090409625 |
| DuckDB        | 0.065306042 | 0.054156167 |
| UDF           | 0.078665125 | 0.080781    |



## Internals

The scalar functions are tracked in a dictionary in the DuckDBHandle
struct. Currently only registering scalar function is supported.

The wrapper is generated via the macro and should be fully type stable.
The wrapper is generated via the function `_udf_generate_wrapper()`.

Because of limitations of the `@cfunction` macro in Julia, the wrapper
needs to be globally accessible. I implemented this by introducing a
global (constant) dictionary variable in DuckDB `_UDF_WRAPPER_CACHE`
which is used to store the generated wrappers. This is defined in
`_udf_register_wrapper()`. This is, in my opinion not ideal but works. I
asked in Julia related forums for a better solution and will update the
code, if something better is possible.


This is an update to PR #14024