Support java.nio.file.FileSystem abstraction

Hi,

Firstly thanks for datoteka I've been using it across several projects for several years.

The main limitation I have encountered in using it however is that it only supports creation of paths on the default filesystem, and doesn't support constructing paths in other java FileSystems; in particular zip files.

In a few projects I've had the need to operate on paths and perform IO reading and writing files in and out of zip files; and consequently I had to work around some of these limitations and develop some custom code for interop with java's ZipFileSystem. Java ships with a ZipFileSystemProvider, which can be used for this purpose.

I was thinking it would be nice to contribute and integrate some of this code into datoteka or as a companion library. Is it something you'd be interested in?

I'm not entirely sure what would be required to maintain backwards compatibility with datoteka yet. However I believe the main change is simply that I added a new `path` constructor which takes a `FileSystem` as the first argument (or if no filesystem is provided here it assumes the default filesystem), and it then returns paths in that filesystem. I believe these paths can then work with most of the datoteka functions as they stand; though some may want to also take an optional filesystem argument.

If you're interested what do you think is the best way to proceed? I was thinking I might first extract the code into a separate library, and share it here for discussion on how best to integrate, before working up a PR to attempt it?

There is some additional code I wrote to also handle reading zip files and managing their resources from multiple threads; which is necessary because the underlying java code doesn't do this very well. I don't know whether that belongs here or not yet; but it is very useful if you need to for example serve files out of zip files over http and may have multiple requests accessing the same file at the same time.

Anyway let me know if this is something worth trying to contribute?

Many thanks.

Inizia esaminando il costruttore dei percorsi di datoteka e le funzioni che operano sui percorsi, quindi confronta le loro assunzioni con FileSystem e ZipFileSystemProvider di Java. Definisci come i filesystem non predefiniti preserverebbero la compatibilità all'indietro e decidi separatamente se la gestione concorrente delle risorse dei file zip debba appartenere a datoteka o a una libreria complementare.

Scritto dal modello di indicizzazione a partire dal testo della issue.