WebDAV: add blocking() to operation which may block

[simonsan]

Code taken from #106 (webdavfs.rs):

#[derive(Clone, Copy)]
enum RuntimeType {
    Basic,
    ThreadPool,
}

impl RuntimeType {
    fn get() -> Self {
        static RUNTIME_TYPE: OnceLock<RuntimeType> = OnceLock::new();
        *RUNTIME_TYPE.get_or_init(|| {
            let dbg = format!("{:?}", tokio::runtime::Handle::current());
            if dbg.contains("ThreadPool") {
                Self::ThreadPool
            } else {
                Self::Basic
            }
        })
    }
}

// Run some code via block_in_place() or spawn_blocking().
async fn blocking<F, R>(func: F) -> R
where
    F: FnOnce() -> R + Send + 'static,
    R: Send + 'static,
{
    match RuntimeType::get() {
        RuntimeType::Basic => tokio::task::spawn_blocking(func).await.unwrap(),
        RuntimeType::ThreadPool => tokio::task::block_in_place(func),
    }
}