loadable extensions

[0xcaff]

integrating the changes from duckdb/duckdb#12682

current issues (needs discussion):

• there are a handful of functions missing from the extensions header (duckdb_row_count, duckdb_arrow_row_count, duckdb_query_arrow_array, duckdb_query_arrow_schema, duckdb_arrow_column_count, duckdb_value_varchar, duckdb_execute_prepared_arrow, duckdb_arrow_rows_changed, duckdb_destroy_arrow). i'm not sure what to do about them. it seems like a bunch of them are part of arrow (idk whether it makes sense to expose that through the extensions api) and the others i'm not sure about.
• how does versioning work in this new extensions api
• idk how the macro works for DUCKDB_EXTENSION_ENTRYPOINT
• do we want one big static object instead of a million small atomic pointers for this

todo:

• figure out bundling for extension headers

related:

• Rust extensions based on Extension C API #370

[0xcaff]

this is a dupe of sam's work, closing in favor of that #370