"smart"er choice(s) for block_size while opening remote file

[yarikoptic]

fsspec's open takes block_size (passed into _open) to instruct on the size of the block to fetch to cover a requested by open range. The default (which we resort to ATM) is 5 * 2**20 = 5M.

• block_size cannot be changed for an already existing sparsely cached file.
• it would depend on use case on what could be the optimal block_size to provide optimal performance
• ... more to come ...