[Feature Request] Dynamically set VARIABLE_IO_THRESHOLD in Prefetcher

[googlyrahman]

See https://github.com/fsspec/gcsfs/pull/818/changes#r3235520806 for more details.

[martindurant]

I would expand the ask here: there are probably a number of defaults that would benefit from considering the connection speed, if we can know it. At the simplest, a test for whether we are running on GCP hardware or not, and then if the bucket is in the same region or not. An obvious example of a good default we want to consider is the file block size.