Better user experience when library is not in shared_preload_libraries

[JelteF]

Related to #200. I think there are (at least) two pain points when using the extension, while not putting it in shared_preload_libraries:

1. If you call read_csv/read_parquet you get an error saying it cannot be executed in postgres. I think this can be fixed by making these functions C functions, then the library will be automatically loaded when you call them.
2. Setting duckdb.execution = true is a no-op. I don't think there's a way to make this experience better, except try to educate people better. Right now the README doesn't mention any of the options to load the library, except for doing "create extension". I think the README should mention LOAD, session_preload_libraries and shared_preload_libraries.