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 <slarengh@gmail.com>

* fix: change `NULL` to `nullptr`

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Co-authored-by: Diego Devesa <slarengh@gmail.com>