cloud_storage: replace empty dir type with fragmented vector

It's typically unexpected for this container to grow large, but we've
seen it before during manual intervention on a node: when trying to
quickly clear out recent data from a cache directory.

(cherry picked from commit 6c59b7a)

src/v/cloud_storage/recursive_directory_walker.cc

@@ -83,7 +83,7 @@ ss::future<walk_result> recursive_directory_walker::walk(
     // Object to accumulate data as we walk directories
     walk_accumulator state(start_dir, tracker);
 
-    std::vector<ss::sstring> empty_dirs;
+    fragmented_vector<ss::sstring> empty_dirs;
 
     while (!state.empty()) {
         auto target = state.pop();

src/v/cloud_storage/recursive_directory_walker.h

@@ -30,7 +30,7 @@ struct file_list_item {
 struct walk_result {
     uint64_t cache_size{0};
     fragmented_vector<file_list_item> regular_files;
-    std::vector<ss::sstring> empty_dirs;
+    fragmented_vector<ss::sstring> empty_dirs;
 };
 
 class recursive_directory_walker {