ggml: load all backends from a user-provided search path (#10699) * feat: load all backends from a user-provided search path * fix: Windows search path * refactor: rename `ggml_backend_load_all_in_search_path` to `ggml_backend_load_all_from_path` * refactor: rename `search_path` to `dir_path` * fix: change `NULL` to `nullptr` Co-authored-by: Diego Devesa <[email protected]> * fix: change `NULL` to `nullptr` --------- Co-authored-by: Diego Devesa <[email protected]>