Make a NetCrsFactory that reacts to actual circuit sizes

[ludamad]

Came out of internal slack thread. Currently, we download for 2**24 circuits in bn254 no matter what in the native bb binary, it would be good to have a common code path between grumpkin and bn254 and cache and download only what is needed based on actual demanded circuit sizes. This would be very similar to the current downloading code, except be wrapped in a crs factory as that is the best way to intercept actual circuit size demands