feat(c_interface): Add object iteration and info functions

[pzittlau]

This commit introduces new C interface functions for iterating over objects and retrieving object information within the betree object store.

• Adds obj_iter_t as an opaque struct for object iterators.
• Implements betree_free_obj_iter to free the object iterator.
• Provides betree_object_iter_all to create an iterator over all objects.
• Adds betree_object_iter_next to advance the iterator and retrieve the next object's key.
• Includes betree_object_store_list_prefix for iterating over objects with a specific prefix.
• Adds betree_object_info_t struct for object status information.
• Implements betree_object_info to retrieve object size and modification time.
• Adds betree_free_object_info to free the object info struct.
• Removes commented-out betree_object_status and betree_object_mtime_us functions.