[Flight] Add read-only fs methods (#20412)

* Don't allocate the inner cache unnecessarily

We only need it when we're asking for text. I anticipate I'll want to avoid allocating it in other methods too when it's not strictly necessary.

* Add fs.access

* Add fs.lstat

* Add fs.stat

* Add fs.readdir

* Add fs.readlink

* Add fs.realpath

* Rename functions to disambiguate two caches