Asyncify filesystem cache

[maminrayej]

In the middle of my investigations for improving startup time, I encountered issue #3851 in one of the comments.
This fix isn't ideal since it uses the tokio thread pool instead of the pool in TokioTaskManager. I couldn't figure out how to design a trait that is both object-safe, and is able to model the spawn_await function that I had in mind properly. This is the best that I came up with:

pub trait AsyncTaskManager {
    type Future<T>: Future<Output = T>;
    
    fn spawn_await<T>(&self, f: impl Future<Output = T>) -> Self::Future<T>;
}

which isn't object-safe.

I could clone the TokioTaskManager directly into FileSystemCache, but I was not sure about it.

[maminrayej]

@Michael-F-Bryan are there any blockers for this PR?

[Michael-F-Bryan]

Let's merge this! 🚀