cachefs: Add InvalidateCacheFS interface and extend cacheFS to implement it

[tmc]

This builds upon #2 to expose an interface for point-invalidation of paths.

This would enable an interface like:

if cfs, ok := fs.(cachefs.InvalidateCacheFS); ok {
    cfs.InvalidateCacheFS("index.html") // error checking omitted.
}

Happy to flesh out some tests here but wanted to get a sense of your take first.