DuckFlight core v0.1.16 — DuckDB 2.0 principal seam

Built from sidequery/duckflight ed89c8dd (PR #242) against DuckDB v2.0.0.

Fixes core-v0.1.15 on macOS hosts that load libduckdb with RTLD_LOCAL (for example Python ctypes or Rust libloading): the principal seam's DuckDB C++ references are now weak on macOS as well as Linux, so the core loads there and runs without the native seam.