Use `tokio::fs` in `HostScanner`

[Molter73]

Since HostScanner runs as a tokio task, we should not be using std::fs operations on it, since it will block the underlying thread and prevent the tokio scheduler from properly doing its job, we should instead prefer using tokio::fs for asynchronous filesystem operations.

The main change for this will be that HostScanner has two RefCell fields, borrowing these results in guards that are not Send, meaning they can't be used through a .await call. We will need to replace them with RwLock or Mutex.

While we are at it, we probably also want to change calls to PathBuf::is_dir, PathBuf::is_file and PathBuf::metadata to instead use a single tokio::fs::metadata call and propagate the result, the individual calls to the PathBuf methods trigger a call to stat under the hood for each occurrence.

[Molter73]

After doing some quick testing, it seems using tokio::fs::metadata causes a new tasks to be spawned for each call and the overhead required to create and destroy this task takes longer than calling std::fs::metadata directly. This pushes me more in the direction that we should move most of our logic to synchronous code and keep only the things that absolutely need async in a tokio executor (Hyper endpoints, gRPC/OTEL outputs, etc..).

I'll keep this issue open to fix the multiple stat calls.