service/fsp_srv: Implement CleanDirectoryRecursively

This is the same behavior-wise as DeleteDirectoryRecursively, with the
only difference being that it doesn't delete the top level directory in
the hierarchy, so given:

root_dir/
  - some_dir/
    - File.txt
  - OtherFile.txt

The end result is just:

root_dir/

src/core/file_sys/vfs.cpp

@@ -384,6 +384,28 @@ bool VfsDirectory::DeleteSubdirectoryRecursive(std::string_view name) {
     return success;
 }
 
+bool VfsDirectory::CleanSubdirectoryRecursive(std::string_view name) {
+    auto dir = GetSubdirectory(name);
+    if (dir == nullptr) {
+        return false;
+    }
+
+    bool success = true;
+    for (const auto& file : dir->GetFiles()) {
+        if (!dir->DeleteFile(file->GetName())) {
+            success = false;
+        }
+    }
+
+    for (const auto& sdir : dir->GetSubdirectories()) {
+        if (!dir->DeleteSubdirectoryRecursive(sdir->GetName())) {
+            success = false;
+        }
+    }
+
+    return success;
+}
+
 bool VfsDirectory::Copy(std::string_view src, std::string_view dest) {
     const auto f1 = GetFile(src);
     auto f2 = CreateFile(dest);
@@ -435,6 +457,10 @@ bool ReadOnlyVfsDirectory::DeleteSubdirectory(std::string_view name) {
     return false;
 }
 
+bool ReadOnlyVfsDirectory::CleanSubdirectoryRecursive(std::string_view name) {
+    return false;
+}
+
 bool ReadOnlyVfsDirectory::DeleteFile(std::string_view name) {
     return false;
 }