prop340: Implementation of packed and fragmented cells

David Goulet requested to merge dgoulet/tor:ticket40791_049_01 into main

This is currently used by congestion control and prop340 will also use it for the relay cell protocol request.

Future commits will use this code with the current congestion control code and new relay cell protover.

Part of #40791

Signed-off-by: David Goulet

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