feat(_ggml): implement ggml-backend API bindings and fix type hints

- Introduces extensive ctypes bindings for `ggml-backend.h` (devices, buffers, registries, and CPU buffer types) to support advanced memory routing like MoE CPU offloading. Also fixes various static typing warnings by adding `# type: ignore` to pointer annotations.

Signed-off-by: JamePeng <jame_peng@sina.com>

Author: JamePeng