Unified loader for ReifyDB extensions, regardless of whether the extension is a native dynamic library, a WASM module, or an in-tree Rust function. Wraps the FFI symbol-resolution machinery, registers callbacks the host provides for the guest, and exposes the typed handles (operator, procedure, function, transform) that the engine uses to dispatch into extension code.
Extension authors do not depend on this crate directly; they target reifydb-sdk instead. This crate is the host
side of that contract - the place the engine looks when it needs to find and bind a registered extension symbol.