Observing changes in the MockFileSystem

[SuperJMN]

Describe the solution you'd like
I'd like to be able to observe common/interesting events (from a testing perspective) that happen into a MockFileSystem.

Why? I'm creating a cache service that uses an IFileSystem and I need to test that a file is only written once.

Describe alternatives you've considered
I've tried to implement my own MockFileSystem using System.Reactive, but the work is a bit overwhelming.

[fgreinacher]

This is the same as #168 which has gone stale.

As mentioned over there I generally like the idea and would accept a contribution here unless it's getting overly complex.