[C-API] Initial support for accessing file system - #19086

This PR adds initial support for accessing DuckDB's filesystem layer from the C-API.

With the current state of the C-API this is primarily useful when writing table functions that scan some external file format, as it allows you to make use of DuckDBs existing cross platform filesystem, as well as any loaded extension file systems (like httpfs), without having to reimplement the equivalent functionality yourself.

I've tried to keep this initial version of the interface as small and simple as possible, while also leaving room to expand it in the future. The most complex part is the error handling, but I think I've solved it relatively neatly by having both the duckdb_file_system and duckdb_file_handle store their last occurred error internally, which can then be converted to duckdb_error_data through a pair of separate functions. Other than that the read/write functions follow posix semantics, returning the number of bytes read/written, 0 on EOF and a negative value on error. In practice the error value is always -1, but I've left it unspecified so that we can potentially return specific error codes in the future (like something like EAGAIN, when we get asyncio), without forcing the user to allocate, inspect and destroy a duckdb_error_data object to handle "common" IO errors.