Support user-defined scalar functions

[jraymakers]

Support bindings and high-level API for all C API functions that use duckdb_scalar_function.

This also requires supporting duckdb_function_info, which is also needed by aggregate, table, and cast functions.

[jraymakers]

Support for most of scalar functions was exposed here: #279
(Underlying support was added to the bindings in several prior changes.)

Still unsupported is the (optional) bind function. Node Neo needs the C API function duckdb_scalar_function_bind_get_extra_info to make this work; this is due for release in DuckDB 1.4.0 (in Sept).

[jraymakers]

Main support released in 1.3.2-alpha.26. Follow-up item filed for bind function support.