Add marker for chained filesystems for path passthrough

[timkpaine]

fixes #1767

[martindurant]

Thanks for working on this!

[martindurant]

Is there any need to explicitly consider async filesystems here?

[timkpaine]

Is there any need to explicitly consider async filesystems here?

I would think this needs to be considered more generically, e.g. what happens if you chain an async fs inside a sync fs? I would think basically no implementations handle this right now. We could consider having

AbstractChainedFileSystem
ChainedFileSystem
AsyncChainedFileSystem

To provide the correct indications via inheritance.

[martindurant]

It might just work already as a synch FS: in the case of the caching ones, it should only call the sync method variants on the target