Support UDF / Custom functions in drivers and SDKs

[avinassh]

libSQL already supports them, but the function creation_function is not exposed in drivers / SDKs.

reported in discord here: https://discord.com/channels/933071162680958986/1218961814302228630

[zicklag]

I'm using the Rust SDK, and I can see I have access to the sqlite3_create_function function in the ffi module, but I don't know how to get the pointers I need in order to use it in the libsql crate.

Does anybody have any pointers on how to go about that?

[zicklag]

I think I'm starting to see how to implement UDFs in libsql. I'm going to try copying how the authorizer is done, mostly.

[zicklag]

I opened a PR for this #2182. Might not be totally ready yet, it only adds SDK bindings for Rust, and still needs support for window functions and aggregation functions, but it looks like it basically works.

This should let me keep using libsql instead of having to migrate to rusqlite so I'm going to try it out in my app and see how it goes.