Add support to register TableFunction and ScalarFunction

[lfkpoa]

Hi,

It would be great to be able to register TableFunction and ScalarFunction using Java.
Many clients in other languages already allow this.
I just found out that C# (.NET) has this and maybe it could be used as an inspiration to implement something similar for java.
https://github.com/Giorgi/DuckDB.NET/blob/develop/DuckDB.NET.Test/TableFunctionTests.cs

Thank you.

[lfkpoa]

I created a PR for this.
#598